Unless means “except if” or “if not.” It introduces a condition that would change what you're saying. When you say “I can't go to the park unless I finish my homework,” you mean that finishing your homework is the one thing that would make going to the park possible.

The word shows up constantly in everyday decisions and rules. “You can't have dessert unless you eat your vegetables” means eating vegetables is the condition that must be met. “We'll play outside unless it rains” means rain is the one thing that would change the plan.

Unless flips things around in an interesting way. Instead of saying what needs to happen, it points to what would stop something or what exception exists. “I'll be there at noon unless my bus is late” acknowledges one possible problem. “You'll do great on the test unless you forget to study” highlights the one thing that could go wrong.

Pay attention to how unless differs from if. “I'll go if you go” and “I'll go unless you don't go” might sound similar, but they emphasize different things. The first focuses on what needs to happen; the second focuses on what would prevent it.