It's an expression of admiration and delight used when you find something very pretty, lovely, or charming.
Here's a breakdown of the phrase:
* Que: "What" or "How"
* cosa: "Thing"
* tan: "So" or "very"
* linda: "Beautiful" or "lovely"
The phrase is often used when someone sees something aesthetically pleasing, like a flower, a sunset, or a piece of art. It can also be used to compliment someone's appearance or personality.