Fan Service is a term used in anime and manga to refer to scenes or situations that are unnecessary to the storyline. This is mostly common in Filler and is usually often something that may exploit a female character in a sexual manner (ecchi) with stuff such as panty shots, bathing suits, bath and shower scenes, unnecessary nudity, etc.
Accidentally falling on top of someone or falling down facing up to see under a skirt is also classic fan service. It is sometimes used with male characters as well such as accidental kissing.
The "fans" who are the target of fan service tend to be heterosexual males, but that's more due to the abundance of female characters in anime.