Other forms: re-entered; re-entering
To re-enter is to come in again after going out, the way you might run back to your car, grab your shopping bags, and then re-enter the grocery store.
To enter is to go in, from a root that means "between or among," and the prefix re- means "again." Whenever you exit a place, you can choose to re-enter it later (unless you're at a concert and there's a sign reading "no re-entry"). A space shuttle that returns to Earth's atmosphere re-enters it, and an injured basketball player might decide to re-enter the game again in the final quarter.