-te oku ??
what kind of grammar construction is it? i read that it's used when you want to say when you do something in advance? how to use it??

Question

-te oku  ??
what kind of grammar construction is it? i read that it's used when you want to say when you do something in advance? how to use it??

Answer

Right. -Te oku implies that you(or subject) do something intensionally, on purpose, in advance for coming situation.

The formula is: te-form of verb + oku.
If you want to conjugate for past tense etc., change "oku", not the main verb.

See the examples:
あさ　うちを　でる　まえに　ばんごはんの　よういを　しておきます。
I prepare my dinner before leaving home in the morning (so I can eat  as soon as I come home).
vs.
あさ　うちを　でる　まえに　ばんごはんの　よういを　します。おふろにも　はいります。そうじも　します。
I prepare my dinner before leaving home in the morning. I also take bath. Also I clean the room.

やまださん、このほんを　よんでおいてください。
Yamada-san, please read this book (because we gonna use it later/ because I'll ask about the book later/because it will help you in future,  etc).

vs.
やまださん、このほんを　よんでください。
Yamada-san,  please read this book (because it was interesting/ because I wrote it/ because it's homework, etc.).

Depending on the context, -te oku may mean to do something and leave it there temporarily (still expecting the coming situation). 
Example:
ちょっと　あついですね。まどを　しばらく　あけておきましょう。
It's a bit hot here. Let's keep the window open for a while
(and close it later).
vs
ちょっと　あついですね。まどを　あけましょう。
It's a bit hot here. Let's open the window.

Hope it helps!

Answer

This is not answer. 
Would you give me sample phases?
Maybe, I can give you answer.