An auxiliary verb attached to the -masu stem to form a (milder) imperative: 'do ...'. Commonly used toward equals or inferiors (e.g., children).
早く食べなさい。冷めてしまうよ。
Eat quickly. It will get cold.
宿題をしなさい。遊ぶのはそれからだ。
Do your homework. You can play after that.
ちゃんと並びなさい。順番を守ってください。
Stand in line properly. Please keep the order.