What Are Or Is Sex Stains?

So, I know its an awkward question, now i hear people say Change the mattress no one will notice, or change the bed sheets when this comes up. Now im not sexually active i havent had sex but, I feel awkward goggling this or asking my parents. So i want to know what are sex stains, I heard about it on "The Maury Show" when woman come out and say "I found sex stains on my mattress".. So yeah. Please no rude comments thanks.