Baby chickens are called chicks. A hen is a female bird. A baby hen could be referred to as a chick. A "flick" means movie. A "chick flick" is a slang term meaning a movie that is geared towards women. "Chick flick" can be taken literally, as in the flick, or the movie, is about chicks, or baby hens. This joke is playing on the slang term "chick flick". 
I watched a movie about a baby hen , it was a real chick flick .