A more thorough explanation:

Extra

Extra means something that is beyond or in addition to what is expected or usual. It can also mean something that is not included or excluded from a group or category.

• She ordered extra cheese on her pizza.
• The teacher gave extra homework to the students who didn't finish their work.
• The store had extra inventory of the popular toy.

The first example shows that the person wanted more cheese than what is usually put on a pizza. The second example shows that the teacher gave more work than what was expected. The third example shows that the store had more of the toy than what was originally planned.