They don't call it heat for nothing. As the weather warms, fertile female cats look to hook up with unaltered Toms. Sixty days later the females give birth to a litter of kittens.
According to the Advocate as many as 50-kittens-a-day can be turned into a shelter. And that means shelters can quickly become overwelmed and need volunteers to help take care of the influx.
Kittens cannot care for themselves and need special attention: Fostering means bottle feeding, stimulating them with a warm cloth to evacuate, teaching how to eat from a bowl, and use a cat box.
At around two months the kittens will be go back to the shelter for vaccinations, getting spayed and then put up for adoption.
Kitten season means more than just fostering. Shelters and animal control agencies need motivated, responsible people to help control the population by means of Trap/Neuter/Return programs.
Many shelters can help with cage traps and free (or low cost) neutering to help with cat populations.