If we are gradually charging it (it means that the provided voltage is within its rated voltage), it will keep on assimilating the charge from the flow of current until it creates a potential equal and an opposite to the supply voltage. Then, because of the capacitor's potential, the current flow can be stopped, and the capacitor charge will remain constant.If we supply a larger voltage to the capacitor, the material present inside the capacitor undergoes a "breakdown," causing it to conduct between the terminals and hence causing the current to flow freely through it.