cat sterilisation

i have joining mailing list which concern about cat. one of the message invite us to sterilize cats. this is my first experience. i wonder what the meaning with cat sterilisation, and then i browse internet and find the answer. sterilisation for cat means to control over cat birth. because uncontrolled cat population, many cat stray being killed every year. it mean, the fewer cats born, the fewer cats have to suffer and die.