In a circuit, charge flows round the loop and carries energy with it. A cell transfers energy to the charge. When the charge passes through a lamp or a motor, it transfers energy from the charge to the component, and the component gives it out as light, heat or movement.
The potential difference (voltage) across a component is the energy transferred per unit charge that passes through it. Charge is measured in coulombs, symbol C. So potential difference tells you how many joules of energy are transferred for each coulomb of charge.
This means the volt is a joule per coulomb: 1 V = 1 J/C. For example, a 6 V battery transfers 6 joules of energy to every coulomb of charge that passes through it. A bigger potential difference means more energy is transferred for each coulomb. Potential difference is measured in volts.