In the positive sense ("until"): You have until Thursday to finish your homework. = Thursday is the last day, you must do it before, not after. In the negative sense ("not until" or "not... until"): Not until I met you, did I know what love really was. = I knew about love when I met you, not before. I won't get home until 8pm tonight because I have to stay late at work. = I will arrive home at 8pm, not before.

Hopefully some examples help: Can we go to the park? Not until you finish eating. Can we leave the park? Not until we finish our game. How long until we go to the park? We won't go to the park until you finish eating. How long until we leave the park? We won't leave the park until we finish our game.