ファン service put simply is stuff the producers do to boost rating usually in もっと見る mature anime. The most common one is revealing a girl's undergarments in some way または form. Why is this bad many great マンガ get turned into animes with to much ファン service. ファン service often can ruin an アニメ and reduce its amount of viewers. Not to mention it also lowers the chance of it ever becoming dubbed.
I will now give examples of some animes that overdid this compared to its manga.
1 rosario vampire in the マンガ it is teen at most parts at least, In the アニメ they put a pantie shot almost every 3minutes which can be really annoying and ruin the epicness of the scenes.
However the worst one of my 一覧 is girls bravo. This アニメ went beyond words of ファン service. The マンガ was about 15 plus and had few シャワー scenes.
In the アニメ there was about 3 bath scenes with nudity per episode making it was mature.
HOPE THIS HELPED THOSE WHO DO NOT UNDERSTAND THE TERM ファン SERVICE.