This is a special tag given in this website to all images that are drawn with the characteristic Japanese, anime drawing style. It does not matter if the picture or the characters are from anime, manga, video game etc.
This special tag is used because this site allows western images to it's database as well. This tag is very important and can be used for the convenience of the users.
If a user wants to 'filter' the images and just view the Japanese ones he can add this tag to his/her search. Likewise if a user wants to just view the western images he can use -anime. See howto:search for more details about subtracting results.