The property of capacitance:

Suppose two very large, flat sheets of metal such as copper or aluminum that are excellent electrical conductors.  Assume that they are each the size of the state of Nebraska, and are placed one over the other, just separated by a foot of space. What will happen if these 2 sheets of metal are connected to terminals of a battery, as shown in Figure given below?

The two plates will become charged electrically, one positively and the other one is negatively charged. You may think that this would take some time, because the sheets are so big. This is an accurate supposition.

If the plates were small, they would both become charged instantly, attaining the relative voltage equal to the voltage of the battery. But as the plates are gigantic,

Figure--   A huge pair of parallel plates illustrates principle of capacitance.

it will take some time for the negative one to fill up with the electrons, and it will take an equal amount of time for other one to get electrons  sucked out. Finally, the voltage between the 2 plates will be equal to the battery voltage, and an electric field will exist in space between the plates.

This electric field will be small at first; the plates do not charge right away. But the charge will increase over the period of time, depending on the size plates are, and depending on how far apart they are. Figure given above is a relative graph showing the intensity of the electric field between plates as a function of time, elapsed from instant the plates are connected to the battery terminals.

Figure--   Relative electric field intensity between the huge metal plates, as the function of time.

Energy will be stored in the electric field. The ability of the plates and of the space between them, to store the energy is property of capacitance. It can be represented by the letter C.