@RedSkyX2
It's a common term which originates from Japanese comedy duo sketches. There are two main characters: "boke", or "idiot one" - a character who says or does something stupid or nonsensical, or just eccentrical, and "tsukkomi", or "straight one" - a "sane" character who responds to boke's foolishness. As it was mentioned by
@Umesan above, it's also a name for any character who points out the craziness going on. Usually main boke-tsukkomi pair in anime and manga are protagonist (harem lead) and his stupid best friend, an ideal example would be Tomoya and Sunohara from Clannad.