What does "menace" mean?
Menace means "This creature can't be blocked except by two or more creatures."

This means when it comes to blocking, your opponent either assigns 2+ creatures to block it or none at all.

You'll see its reminder text on cards from other editions like Insolent Neonate, but it looks like none of the Menace cards in Kaladesh had reminder text for the ability. Menace became a keyword ability during the Origins set.