Ground, grounding, and grounded, are real tangible physical things and processes as well as mathematical idealizations.
In the real tangible physical sense, grounding refers to the process of connecting a conducting wire usually made of metal (copper is a great choice) to a conducting terminal on an electrical device. The other end of this wire is firmly attached to a conductor (again think some metal) that is implanted into the ground (literally the thing that we walk on Earth). Some pictures of what this looks like is here:
https://en.wikipedia.org/wiki/Ground_(electricity)
Once this connection is made, we say that the electrical device is grounded.
Their are many purposes of this grounding wire and this grounding process:
- Electrical devices sometimes measure electric potential or produce an electric potential. In either case, this electric potential is actually a potential difference between the two points. One of these points is a reference point. The grounding wire that connects the electrical device to the conductor implanted into the ground is the reference point. In electrostatics, a conductor is an equipotential surface. Therefore, all points from the connection point on the electrical device, through the grounding wire, and to the connection point on the metal implanted into the ground are all the same potential (i.e. the potential difference between any two points on these objects is zero). The reference point is therefore really the metal implanted into the ground (if we are in the electrostatic regime or at equilibrium - i.e. no charges are moving anymore). The word ground is generically used to refer to anything that is in contact with and has zero potential difference with the reference point (the conductor implanted into the ground).
- Electrical devices that generate an electric potential difference have essentially placed and maintain a net charge on a formerly neutral object. This charged object is usually a conductor since is it essentially cost free (in terms of energy) to move electrical charges through and within a conductor. If a formerly neutral object becomes charged, then this net charge had to have come from somewhere. This somewhere is ultimately the Earth (literally the thing that we are standing on) via the conductor implanted into the ground. The Earth is essentially an infinite supply of electrons. We’ll come back to this very subtle point a little later because it has consequences for how reliable the conductor implanted into the ground is as a reference point. Finally, we’ll note that a positively charged conductor has had some electrons removed from it and transferred into the Earth through the grounding wire and via the conductor implanted into the ground. Likewise, a negatively charged conductor has had some electrons added to it and transferred from the Earth through the grounding wire and via the conductor implanted into the ground.
It is cumbersome to keep using the phrase “electrical potential difference with respect to a reference point defined by a conductor implanted into the ground.” As a consequence, we simply use the word potential (fewer letters to type) and you as the reader need to know that all of the other words in the original phrase is implied.
There is an direct relationship between some object having some nonzero potential and having some nonzero charge on it. The relationship between the total net charge on an object and the potential that the object is held at (relative to some reference point like ground) is complicated but only ultimately depends only on the geometry (think sizes, shapes, and distances) and intrinsic material properties (i.e. the dielectric constant for that material). For the special case of a conductor in electrostatics, the potential V everywhere on the conductor is the same and this potential is related to the total charge Q on the conductor by its capacitance, C = Q/V:
https://en.wikipedia.org/wiki/Capacitance
The capacitance tells us how much the potential changes on a conductor if charge is moved onto or away from the conductor. This seems to imply that, if charge is moved from or to the conductor implanted into the ground, then the potential on the conductor is changing. That seems concerning because the potential on the conductor implanted into the ground is supposed to be the reference point for measuring and creating potential differences using our electrical devices. If we want to be measuring or creating potential differences that have a particular time dependence, then it is highly desirable that our reference point has as little time dependence as possible. However to create a nonzero potential difference relative to ground, we have to use our electrical devices to moves charges to and from ground which seems like it would change our reference point.
The solution to this problem is choosing a ground that has an infinite supply of electrons. This is not possible and is therefore a mathematical idealization. We call this magical mathematical idealization of ground “ground.” Yeah I know, this is confusing. To recap: ideal ground and real ground are both called ground. Back to the real world, the Earth (literally the thing that we are standing on) is effectively a source of 10^{45} electrons (by my super sketchy estimate). If we were to move an Avogadro’s number of electrons onto or off of a grounded conductor, the relative change in the Earth’s electron supply would be just one part in 10^{22}. The Earth and the real ground that we stand upon is therefore a very very good approximation for ideal ground. Another way of saying this is that the capacitance of the Earth is infinite. Any change in the number of electrons in Earth is so small that the potential of Earth is “rock” steady and it is indeed an excellent time-independent reference point that we can use to precisely measure and create electric potential differences against.