The rectangular area on each side of the net where a serve must land. There are four service boxes total, two on each side, divided by the center service line. Each box is 21 feet deep and 13.5 feet wide. If your serve doesn't land in the correct diagonal service box, it's a fault. Seems like a big target until you're standing on the baseline with a racquet in your hand.
"First serve lands right in the middle of the service box. Not a great serve, but it's in."