A fetish is an extremely strong devotion to something. There are sexual fetishes and nonsexual fetishes: both are obsessive interests.
The most common use of the word fetish is probably the sexual meaning. Someone with a foot fetish is abnormally interested in feet and gets sexual pleasure from seeing, being around, and even thinking about feet. There are probably thousands of sexual fetishes, most of which are too inappropriate to write about here. A non-sexual fetish is just an excessive interest in something, like a football fan who lives and breathes everything NFL.