A colloquial expression meaning 'I have to go' or 'I've got to be going.' It's a shortened form of '行かなければ' and commonly used in spoken Japanese.
もう遅いから、そろそろ行かなくては。
It's getting late, so I should be going soon.
授業があるから、行かなくちゃ。あとで連絡するね。
I have class, so I have to go. I'll contact you later.
もう行かなくてはならないので、失礼します。
I must go now, so excuse me.