Netflix is a service providing TV shows and movies without commercials to your home.
"Netflix and chill" means to watch TV shows and movies together.
However, it has a sexual connotation. If a man invites a woman to "Netflix and chill", it can be interpreted that he is not interested in watching a movie, but rather in sexual intercourse. He will put the movie on the screen and then proceed to "make his move" (cuddle up against the woman, try to kiss her, invite her to have sex).
Recently, it is a joke on the internet that "Netflix and chill" means sex.
I hope this answers your question :)