팬 service put simply is stuff the producers do to boost rating usually in 더 많이 mature anime. The most common one is revealing a girl's undergarments in some way 또는 form. Why is this bad many great 망가 get turned into animes with to much 팬 service. 팬 service often can ruin an 아니메 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 망가 it is teen at most parts at least, In the 아니메 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 아니메 went beyond words of 팬 service. The 망가 was about 15 plus and had few 샤워 scenes.
In the 아니메 there was about 3 bath scenes with nudity per episode making it was mature.
HOPE THIS HELPED THOSE WHO DO NOT UNDERSTAND THE TERM 팬 SERVICE.