Trap, neuter and return is the process of sterilising stray animals and placing them back in the environment where they came from.
While many people feel that it is inappropriate to return sterilised animals to their origins (they would much rather have them removed), these animals DO have an important function: they keep the population under control. Imagine that somehow an island would be vacated by its original inhabitants. It wouldn't be long before an entirely new generation of inhabitants would arrive, but this time uncontrolled. So it is better to control a population through sterilisation, vaccination, deworming and medical care so that it will hold its ground against influx.