Tonight means the evening and night that are part of today, the period between now (or this afternoon) and tomorrow morning. When your teacher says “finish your homework tonight,” she means before you go to bed this evening. When a family plans to watch a movie tonight, they mean later today, after dinner perhaps, not tomorrow night or last night.

You might tell a friend “see you tonight” if you're meeting them this evening, or hear a weather forecaster say “expect rain tonight” if storms are coming after sunset.

Notice that tonight only works for the present day. Once you wake up tomorrow, what was “tonight” becomes “last night,” and there's a new tonight ahead. If you're making plans for the evening two days from now, you'd say “the night after tomorrow,” not “tonight.”