What are leaves?
A leaf, or leaves, are sheets of paper in a publication.
What is a side?
A side refers to a printed page.
What is a page?
A page is represented by the side of a sheet of paper. One side of the leaf is one page and the other side is another page.
So if you opt for single-sided printing, one side counts as one page. If you select doublesided printing (i.e. you want your work printed on the front and back of a single leaf) then that will count as two pages or two sides.