Until means up to the time when something happens or up to a particular point. If your mom says “you can play outside until dinner,” she means you can keep playing right up to dinnertime, but you need to stop when dinner is ready. If a library book is due on Friday, you can keep it through Thursday, but it needs to be back by Friday.

The word marks an endpoint or boundary in time. When you say “I didn't understand fractions until Mrs. Chen explained them,” you mean that understanding came at a specific moment, and before that moment, you were confused. When a coach says “keep practicing until you get it right,” she means don't stop before you succeed.

Until is different from by, though people sometimes confuse them. “Finish your homework by 8:00” means it needs to be done at or before 8:00. “Work on your homework until 8:00” means keep working up to that time, even if you're not finished. That's an important distinction when following instructions.

You'll also hear people say “up until,” which means the same thing but adds emphasis, as in “up until yesterday, I had never tried sushi.”