「入る」(hairu) means to go into or enter a place, such as a room, building, or bath. It's an intransitive verb, so it doesn't take a direct object. You use the particle "に" to indicate the place you are entering. For example: 部屋に入る。(Heya ni hairu.) = To enter the room.

ある場所の中へ移動することです。「〜に入る」の形で使います。例：お風呂に入る。