A "moat" is the protection company A has against company B building a clone of company A's product. For example, YouTube has the set of people who have a habit of logging on to YouTube to watch content. A competitor has to convince all those people to change their habits. This is known to be a hard problem, so it's a moat!

Netflix thought they had a similar moat of user-habits. But it turns out that the more important moat in the streaming market is a moat of content.