It is an adverb that means "as much as possible". It is used when describing situations in which you try to do something as much as you can.
To stay fit, I go to the gym every other day and try to have a well-balanced diet.
健康の為、一日置きにジムに行き、なるべくバランスのとれた食事をとる。
健康の為、一日置きにジムに行き、なるべくバランスのとれた食事をとる。
Amy and Meg are talking about the schedule for the tomorrow’s party.
- Amy : What time will you go to the restaurant?
何時にお店に行く？ - Meg : I'll get off work at 6 so I'll be right over.
6時に会社を出て、向かうね。 - Amy : Okay. Please come as soon as possible.
わかった。なるべく早く来てね。
Meg and Any are talking about the temperature in their room.
- Meg : The temperature in this room is too hot, isn't it?
この部屋、暑過ぎない？ - Amy : I'm trying to use the air conditioner as little as possible to avoid feeling cold.
体が冷えないように、なるべくエアコンを使わないようにしているの。