It means they have cash in a bank account ready to go. Maybe they took out a loan against other assets to obtain the cash, but a "cash offer" is literally the person is handing over a cashier's check at closing.

This means there is 0% chance of the deal falling through. Not almost no risk, none at all. Failure to follow through with the deal would be breach of contract and the buyer is liable for damages, up-to-and-including, being forced to hand over the money for the house.