how does capacitor work ? how do they conduct electricity ?
It's like a flexible diaphram in-line with water flow in a pipe that conducts water.  It can't indefinitely, but it can allow water to flow back and forth a limited amount.

The insulator in a capacitor is like the diaphram in that no charges can pass.  However, charges do pile up on both sides of the insulator so that current can flow up to a certain point, or back and forth current can flow.

The amount of charge that has to build up to get to a certain voltage (pressure across the diaphram) is proportional to that voltage.  That proportionality constant is the capacitance.  A higher capacitance is like a looser diaphram in-line with water flow.