In an electrical circuit the cell (or battery) pushes the electrons around the circuit. It does this by transferring chemical energy from the materials in the cell to electrical potential energy to the electrons.
When electrons pass through a lamp in the circuit they give up (lose) some of the electrical potential energy to the thin wires in the lamp in the form of heat and light. Therefore across the lamp there is an electrical energy difference. The electrons entering the lamp have a higher electrical energy than the electrons leaving the lamp. This difference in electrical potential energy across the lamp is called a “POTENTIAL DIFFERENCE or p.d.”
It is for this reason potential difference is sometimes also referred to as voltage.
By connecting a voltmeter in parallel across the component of interest the potential difference across it can be measured (in this case the lamp)