Yeah, I'm a libra...

 

Here is an old meme. It is a poker face meme, which is generally a meme where someone tries not to reveal information about something by making a neutral facial expression. They tend to use the image in the lower right quadrant as part of them.

The male person here was caught in a lie, and in the last panel he is trying to not look surprised because his birthday is either not in the range of a birthday that would make him a libra, or (perhaps more likely) he has no idea what birthday he could say to make him a libra.

People being caught in a lie are funny, and that is why this is funny.