Spay and neuter (generally called castration) is the process of rendering an animal sterile, thereby removing the ability to reproduce.
Thousands of kittens and puppies are born every day to stray mothers, or at least, mothers that receive little or no care from humans at all. Needless to say, these animals must fend for themselves and in doing so, they may offend humans that live in their surroundings. Humans often treat such stray animals without respect, or worse, actively pursue them. Therefore the best way to help animals is to make sure that they can't reproduce.