Refers to the evening or night of the current day. Often used adverbially without a particle. Compare 今晩, which is slightly more formal and less common in casual speech.
See also: 今晩
The moon is beautiful tonight.
I'm going out to eat with a friend this evening.
I'm going out to dinner with a friend tonight.
I have to finish the report by tonight.
I'm planning to stay at a hotel tonight.