「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(ばしょ)の中(なか)へ移動(いどう)することです。「〜に入(はい)る」の形(かたち)で使(つか)います。例(れい)：お風呂(ふろ)に入(はい)る。