Voltage is the electrical potential difference between two points. It is often compared to pressure because it helps push electric charge through a circuit.
How voltage is measured
Voltage is measured in volts. A battery, power adapter, solar cell, or generator can provide voltage. A multimeter can measure voltage between two points in a circuit.
Voltage is not the same as current
Voltage is the push. Current is the flow of charge. A circuit needs a path and a load for current to flow.
Voltage tells you the electrical difference available, but it does not by itself tell you how much current is flowing.