A difference in electrical potential between two different places.
The term voltage is going to be used a lot throughout your training. You would do well to understand voltage. Not having a good understanding of this term and what it is will impede your ability to understand almost everything else. So many apprentices don't get this. Learn what it is. Learn how to measure it. Learn how it drops and it's value in different parts of a circuit. Know this, and so much other stuff is easier to understand.