It's a common way to express admiration for something that is pleasing to the eye or otherwise evokes a positive emotional response.
Here are some examples of how it can be used:
* "Que bonito vestido!" (How beautiful dress!)
* "Que bonito paisaje!" (What a beautiful landscape!)
* "Que bonito gesto!" (What a beautiful gesture!)
It's a very versatile phrase that can be used in many different situations.