General: one's virginity.
Specific: refers to the blood left on your sheets after you pop a girl's cherry. The sheets are from that point forward the girl's V-flag, you have captured it and taken it to your home base (or can be run up a flag pole).
"Dude, I captured that angel's V-flag."
"No way, she was waiting til marriage."
"Believe this. Look up the top of the flag pole."