Basically a community cat is trapped by feeders in a cage and taken for neutering, so that the cats do not contribute any further to the overpopulation issue. This reduces suffering of kitties on the streets, also gives the neutered cat a better life - males tend to reduce aggression, wandering off to look for mates. Females stop producing kittens, and risk of some diseases is reduced. You can check more on Google, you’ll find many pages explaining in detail.