A metal fish decal that parodies both the Christian Jesus fish and the Darwin fish that lots of people put on the back of their car. Since so few people know what Linux is, the decal confuses many passersby. It’s intentions are not as discernible as the Darwin fish so it doesn’t draw anger from Christians. It is not directly poking fun at Christians but rather it is an inside joke between Linux advocates and a snub towards Microsoft. It’s sort of a secret handshake between fellow devotees to the Linux computer operating system. It's jokingly saying that Linux can save the world.
Hey, that dude has a Linux fish on his car. He must be some kind of computer geek or something that hates Microsoft. I bet the Atheists and the Christians don't have a clue what his fish is about.