She chewed on her hair, an old habit of hers that she was trying to break.
The second half of that sentence is “telling” the reader about her old habit. There’s a more active way to “show” it by incorporating her emotions into it. For example:
She caught herself chewing her hair and dragged it out of her mouth. Nasty habit. Why couldn’t she break it?
In the example, I’ve delved deeper into her point of view and “shown” the same information in a more active, emotional way.
This might seem like a rather trivial example for “telling,” but think about if you changed all these instances throughout your manuscript. The small changes made would contribute to the whole of the writing, making it just a bit crisper and sharper.