The verb "ought", according to Wiktionary, is a verb that is used to indicate duty or obligation, advisability or prudence, desirability, or likelihood or probability. For example, "You ought to go home early" means "You should go home early" or "It would be better (for you) if you went home early". "Ought" doesn't get used as much as "should", "may", "must" and others.
"That ought to do it" is probably the sentence where "ought" is used most often. Sometimes you'll hear it pronounced as "That oughta do it". It means "That should do it", "That should be enough", or "That should be sufficient". For example, a mechanic fixing a car might decide that he has done enough work to fix it. He might then say "That ought to do it." Or if someone is going on a long journey and the gas attendant is filling his car, the driver might tell him "That ought to do it" when he is satisfied his car has enough fuel.