Wikki explains it quite well.
Briefly she spoke to the house of democrats saying that contraceptives should be provided by university health insurance. She had a friend with PCOS, the insurance company would not cover contraceptives even for medican non contraceptive use.
Limbaugh thinks that makes her a 'slut' and a 'prostitute'.
en.wikipedia.org/wiki/Rush_Limbaugh%E2%80%93Sandra_Fluke_controversy