= to make certain that someone knows the real facts about a situation
set straight vt.打扫，纠正。

【例句】
Don't worry, I set him straight (on this matter).
不用担心，我（把这件事）跟他说清楚了。
 
It seems that you are still confused. Let me set you straight.
看来你还是困惑。让我来给你解释清楚吧。