When you straighten something, you either unbend it or tidy it up. So you might straighten your legs, get off the couch, and straighten the room.
To make something straight is to straighten it. When you uncross your legs and stretch them out in front of you, you straighten them. And if you adjust a picture on the wall so it's perfectly level, you straighten it. In the the 14th century, the equivalent verb was straight — so in those days, your mom would have said, "Straight your bedroom before you go out with your friends!"