It is an operation carried out on an animal's reproductive organs to prevent it from producing offspring.
The procedure is performed under general anaesthetic, so the animal feels no pain. There may be slight discomfort for one or two days afterwards, but it is soon over, and your pet's chances of a healthy life are enhanced.
It is not true that a sterilised animal will become fat and will no longer be playful. Your vet will advise if you need to cut back on the amount of food given to your pet. Many pets become more playful and affectionate because they're not distracted by the urge to mate.
If you think it is cruel or unnatural to sterilise your pet, think again. It is even more cruel to deny unsterilised animals normal mating. Sexually frustrated animals not only become restless and call out constantly, they can also lose weight and become susceptible to illnesses. If you allow them to mate however, you could be contributing to the problem of unwanted animals in Singapore. A sterilised animal will be a calmer and easier pet to look after.