Ear tipping indicates a free roaming or feral cat as having been spayed/neutered and vaccinated.
If you notice a cat with one ear that is flat across the top as pictured here, it indicates that the cat has been detained, taken to a spay and neuter clinic, sterilized to make sure they cannot produce more kittens and returned to where they were located in the first place. This is done to help prevent more unadoptable or unhomed kittens in homes of reducing the population of free roaming cats in every neighborhood, by stopping the reproduction thereby decreasing the numbers.
Recognizing an ear tip spares cats from being transported, held, brought to a clinic, sedated, explored, transported and held even longer before their return.
Please help to educate your friends and neighbors that these ear tipped cats are NOT adding to the overpopulation and should be left alone. Do NOT turn them into shelters where they may not be adoptable but rather euthanized. Most have caretakers who manage colonies of cats and check on their well being and tend to them offering shelter, water and food.
If on the other hand you notice a stray without an ear tip, contact your local animal rescue groups to find help for them.