What is a meme or me me ?

I was trying to find out where the expression “Jesus Christ! It’s a lion!Get in the car!” comes from. I first encountered it when purchasing a fridge magnet showing a cat in a car. The picture had me laughing for days. Looking it up in Google I have now found the tragic background (a tourist eaten by a lion). Several sources called the expression (Jesus Christ!) a meme. I now know how to pronounce it, but what does it mean ?