TNR stands for Trap-Neuter-Return, the following snippet lets you put a toe in the water but we suggest reading the whole article.
A teacher at the high school, your boss, the clerk at the grocery store—people all over the world care for outdoor cats every day. Like them, you’ve discovered cats in your community, and you want to help them.
The cats you are seeing are feral cats. Feral cats are domestic cats. Like pet cats. The difference is that feral cats have had little or no contact with humans, and so they are fearful of us and cannot be adopted. They have a home—the outdoors.
And, there is a simple way you can help them: Trap-Neuter-Return. This program ends reproduction, stabilizes feral cat populations, and improves individual cats’ lives. The behaviors and stresses associated with mating— pregnancy, yowling, and fighting—stop.
What is Trap-Neuter-Return?
Trap: Humanely trap all of the cats in a colony (a group of cats living outdoors together).
Neuter: Take the cats in their traps to a veterinarian or clinic to be neutered, vaccinated, and ear-tipped (a universal symbol indicating they have been neutered).
Return: Return the cats to their original outdoor home.