When the conjunction particle て is followed by the う-Verb 置（お）く in Japanese, it regularly has the meaning of ‘to do (A) in advance’. 置（お）く as a verb itself means ‘to place something’ (down), but also has the nuance ‘to leave something alone’, or to ‘drop something’ (like a topic). In this way, ておく can be thought of as meaning ‘to do (A) and leave it (until later)’.

• 寝（ね）る前（まえ）に食器（しょっき）を洗（あら）っておく。
  Before sleeping, I will wash the dishes in advance. (So that it does not need to be done later)
• あなたの財布（さいふ）を鞄（かばん）に入れておきます。
  I am going to put your wallet in your bag. (So that you won't forget it later)
• 予約（よやく）しておくね。
  I am going to make a reservation in advance. (So that the bookings don’t fill up while we wait)

When using ておく, the ‘future event’ that the (A) action is being done for may be something that is coming soon, or may be something that the speaker doesn’t want to ever come. Due to this, ておく is also regularly used when the speaker is doing something specifically to prevent another thing from happening.

• 毎日（まいにち）歯（は）を磨（みが）いておくと、虫歯（むしば）が防（ふせ）ぎます。
  If you brush your teeth everyday, you will be able to prevent cavities. (That may or may not come in the future)