This structure uses the te form with the verb “to put” to express that you will perform an action and then leave it in that state (typically in preparation for something in the future). For example, you might have guests coming in a few hours so you say to your partner, “I’ll put the wine in the fridge” or you might be late for a meeting so you give someone a call to let them know in advance. It doesn’t always have a direct equivalent in English.
準備しておく。
Prepare ahead.
遅れそうだからジェームズに電話しておくね。
It looks like we’re going to be late so I’ll give James a call.
ビールを冷やしておく。
To cool the beers.
お客さんが３時ぐらいに来るので、会議室を予約しておきましょう。
The clients are coming around 3 o’clock so let’s book a meeting room.
予備の電池を用意しておく。
Keep a spare battery.
In casual conversation, the ～ておく sometimes gets abbreviated to 〜とく.
準備しとく。
Prepare ahead.
買っといてよかったよね。
It’s a good job I bought it [because the price has now gone up].