Answer:
On island 1, finches with short, strong beaks would be more likely to live and reproduce, and pass on the trait - the short, strong beak - to their offspring. On island 2, however, finches with long, thin beaks would be more likely to live and reproduce, and pass on the trait - the long, thin beak - to their offspring... Natural selection then is the process by which individuals with useful traits are more likely to survive and produce offspring, which they pass on the useful traits to.
Hope that helps! Good luck! <3