This happens when a woman loses her virginity.
This allusion is derived from high-end electronic products, such as hard drives and 6th-generation-and-newer video game systems, where there is usually a seal which states that the manufacturer's warranty will be void if it's damaged or removed. The damage or absence of this seal will tell whether the product has been internally tampered with.
Likewise, a woman also has a seal, the hymen (which everyone usually calls the "cherry".) When a man plows through it with his meat missile, she's not a virgin any longer. The seal is broken, and her "warranty" is voided.
I heard the star quarterback took the head cheerleader out on a date this weekend... They parked down at "The Point", one thing led to another, and she got a voided warranty, if'n you know what I mean. But that's nothing compared to that geeky guy in the science club -- he managed to get laid here on campus, in the storage area! He's a LEGEND!