A word filter is, as the name implies, a filter for words. Various websites too fucking pussy to use proper internets english will use these strange devices to change one word into another. Example: faggot->homosexual, rape->tenderly caress, etc. Other websites (ie those that do not suck) will use them for more humorous purposes. 7chan is a prolific user of these, changing all manner of words for no real reason. For example, faggot filters to spider expert, while god filters to Bob Ross. Also known as cheesegraters.