Help to Diagnose a Problem with Northrock XC6 Mountain Bike
Cassette sprockets are highly worn and chain is slipping off of sprocket teeth, or (and more likely I think) the freehub freewheel mechanism is slipping, either due to pawls not extending properly or damage to pawls or body.

Sometimes freewheel mechanisms get gummed up with dried out grease which makes the pawls stick. Liberally applying solvent can sometimes unstick them, but I'd look into disassembly and regreasing.