From Longman Dictionary of Contemporary Englishthat’s the ideathat’s the ideaspoken a) PRAISEused to tell someone who is learning to do something that they are doing it the right way, in order to encourage them Keep your knees bent and lean forward slightly. That’s the idea! b) used to emphasize what the main point of something is, or to say that someone understands that point ‘You’re thinking of getting a new job?’ ‘Yeah, that’s the idea.’ → idea
Examples from the Corpusthat’s the idea• Grown-up amusements, that's the idea.• And that's the idea behind Cotswold Garden tours, which organises holiday breaks in the region.• Now push the button on the left to set the time. That's the idea!• "You're going to meet them there?" "Yeah, that's the idea."