「やめとく」は「やめておく」を短くした話し言葉で、何かをしないと決めた時に使います。

"やめとく" is a shortened spoken form of "やめておく," used when deciding not to do something.

友だちとの会話でよく使い、少しやわらかい言い方になります。

It is often used in conversations with friends and sounds a bit softer.