fã service put simply is stuff the producers do to boost rating usually in mais mature anime. The most common one is revealing a girl's undergarments in some way or form. Why is this bad many great mangá get turned into animes with to much fã service. fã service often can ruin an animê 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 mangá it is teen at most parts at least, In the animê 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 list is girls bravo. This animê went beyond words of fã service. The mangá was about 15 plus and had few chuveiro scenes.
In the animê there was about 3 bath scenes with nudity per episode making it was mature.

HOPE THIS HELPED THOSE WHO DO NOT UNDERSTAND THE TERM fã SERVICE.