Something absurd is really silly, absolutely ridiculous, or total nonsense. Thinking you can wear flip flops and a bikini to the North Pole is an absurd idea, for example.
If you run into someone dressed in an absurd outfit or watch a movie full of absurd jokes, you'll probably have a good laugh. But find yourself dealing with the absurd; you'll likely be stuck in a depressing existential crisis. The absurd describes a state of being where human life has no purpose and everything is completely irrational.