A moat is the trench around the outside of a castle that is often, but not always filled with water. It's a defensive structure because people have to cross it to get your castle in the middle. So Uber's castle (i.e. it's product) doesn't seem to be that well defended against competitors (it has a weak moat). They have the numbers and the user adoption, but fundamentally they're no different from Lyft et al.

That's my interpretation anyway! I've never heard 'weak moat' used in this context before though. Apologies if you actually know what a moat is and that all sounded rather patronising :/