It's a way of expressing a desire or a longing for something that isn't possible or is unlikely to happen.
For example:
* "Lla quisiera tener más tiempo libre." (I wish I had more free time.)
* "Lla quisiera ganar la lotería." (If only I could win the lottery.)
The phrase can also be used to express a strong desire or a yearning for something that is difficult to achieve.
* "Lla quisiera poder volar." (I wish I could fly.)
* "Lla quisiera hablar con ella." (If only I could talk to her.)