Colloquial contraction of 〜ておく / 〜でおく meaning to do something in advance or prepare/leave something as is for later.
先に宿題を終わらせとくね。
I'll finish the homework ahead of time.
写真は撮っといたほうがいいよ。
You'd better take a photo (in advance).
出かける前にガスの元栓を閉めといて。
Please turn off the gas main before you go out.
忘れないようにメモしとく。
I'll make a note so I don't forget.