They remove selected parts of the signal. EQ is a type of filter, so is wah, so is flanger, so is the tone control on a passive bass. I would give you a more specific answer if you would ask a more specific question.
Take a look here. It's an article explaining the technical facts about bass guitar amplification. There's a section about filters. http://www.talkbass.com/forum/showthread.php?threadid=24546&highlight=AND+read+AND+faq