Sorry, one cannot convey a proper answer in a simple statement or explanation because the study and behavior of capacitors is usually covered in a Year I or Year II course in circuit theory.
If you told use your age and level of education we can provide a clearer answer for your purposes.
Some common uses are noise filtering, power supply ripple reduction, filtering or shaping signal frequency response, blocking DC in AC signal paths, energy storage.
A capacitor stores electrical energy in the form of a charge separation, which appears as a voltage difference across its poles. All of its uses derive from this basic function.