A capacitor is an electrical component built from two conductive plates separated by a thin insulating material, storing electric charge on those plates when a voltage is applied across them. Its capacitance, a measure of how much charge it can store for a given applied voltage, isn't a fixed universal property of capacitors in general — it depends directly on the capacitor's own physical construction, specifically the plates' surface area, how far apart they're separated, and the particular insulating material, called the dielectric, sitting in the gap between them.
Applying a voltage pushes opposite charge onto each of the two plates
When a voltage source is connected across a capacitor's two plates, it drives positive charge onto one plate and an equal amount of negative charge onto the other, with the intervening insulating material preventing that charge from simply flowing directly across the gap and neutralising itself. The separated charge sitting on the two plates creates an electric field across the gap between them, and this stored charge and the resulting field persist even after the capacitor is disconnected from the original voltage source, at least until some path is provided for the charge to discharge back out.
Plate area, separation and the dielectric material all shape how much charge fits
A capacitor's capacitance increases with greater plate surface area, since a larger plate surface can accommodate more separated charge for the same applied voltage, and it decreases as the separation between the plates increases, since a wider gap weakens the electric field's ability to hold the separated charge in place efficiently. The dielectric material filling that gap also matters directly: different insulating materials have different abilities to sustain an electric field without breaking down or allowing charge to leak across, and materials with a higher relevant property, called permittivity, let a capacitor of otherwise identical physical dimensions store meaningfully more charge for the same applied voltage.
What we're still unsure about
The basic physical relationship between a capacitor's geometry, its dielectric material and its resulting capacitance is precisely defined, settled classical electromagnetism, well confirmed across an enormous range of practical applications. What remains more genuinely an active area of applied materials research is developing dielectric materials that can push a capacitor's achievable capacitance and energy density even higher within a given physical size, which matters directly for applications like energy storage in electric vehicles and renewable power systems — researchers continue actively working on new dielectric materials and capacitor designs, without there yet being one settled, universally optimal material or structure that maximises energy density for every application at once.
This sits inside Electric Potential & Capacitance, one of eight topics in Electromagnetism, one of five domains in Physics, one of seventeen subjects the app can quiz you on.