The term "manspread" is very colloquial and would be classed as slang. It basically means when a man is sitting with his legs spread wide apart. So instead of saying "the man is sitting with his legs spread open", you can say "he is manspreading".

As Siobhan says, 'manspreading' describes a man sitting with his legs wide apart, taking up a lot of room. This term is generally used in the context of public transport and other shared public spaces, and 'manspreading' is seen as a selfish way to behave. A person sitting on a crowded bus or train with his legs spreading into the territory of the neighbouring seats is sure to make himself unpopular with fellow passengers.

I'm a native speaker and I've never heard the term. I would not use it.

"Manspreading" is, what people call, a portmanteau. That is a union of two words that makes a third one with a different meaning.