To "pop your cherry" is to lose your virginity.

It's a reference to the breaking of the hymen that some girls have.
When the thin piece of skin, that partially occludes the entrance to the vagina, tears.

Some girls don't have a hymen to begin with, others tear or stretch theirs during sports, or horseback riding or using tampons or even when self masturbating.

Losing your virginity is when you have penetrative sex (ie penis into vagina).

So if you have "done it" a lot then you have had your cherry popped.