Straightening means making or becoming straight. It can also refer to the act of fixing something that's bent or crooked.
When we talk about straightening, usually we mean using some kind of force or effort to make something straight again. For example, if you have a piece of wood that's warped from being left outside in the rain, you might need to use a tool to straighten it out so it can be used for your project. You could also talk about straightening your hair after it gets all frizzy and messy on a humid day. If you're a fan of ironing clothes, you know that's another way of straightening things - making wrinkled fabric smooth and flat.
To cause to become straight.
The act of making something straight.
