Castration is a (relatively uncomplicated) surgical intervention. Tom cats lose their testicles in this and female cats get their ovaries cut so that they are not able to give life to any new kittens.
Testicles and ovaries are gonadals that control, together with certain brain areas, the cats' sexual drive by means of hormone releases.
When those gonadals are being removed or disabled from one day to the other without a word of warning to the cat. That, of course, is pretty confusing for the cat and the hormonal balance.
The change of the hormonal balance usually leads to an increased appetite. The first two months after the castration are here the critical time in which some tom cats who are living with careless cat owners take on a third of their original weight. It doesn't seem much at first when a cat gains a weight from 8 to 12 pounds, but on the other hand a human who gained weight from 120 pounds to 180 seems quite fat to us afterward.
But the opposite of this scenario also happens in more cases than one would think: Some tom cats (female cats are much more "immune" to it) show an alarming loss of appetite after their surgery. That also is caused by the unbalanced hormone status that confuses both the cat and also its entire body.
In most cases this lack of appetite is harmless. If a cat, though, doesn't touch her water, is vomiting frequently, shows a strong exhaustion or doesn't eat for two days you should see the vet another time, because the surgical scar might be infected.
The general lack of appetite should be gone after two months usually and your cat will have a usual lazy cat life.
You might also be interested in the following articles: My cat doesn't eat anymore, Encourage your Cat to eat again, Force feed your Cat.
Picture CC 2.0 by pmarkham