The meaning of the word incredulous, based on its parts is: "full of disbelief".
The prefix in- is one of the many prefixes that mean "no". So it denies the meaning of the word following it. One example would be the word inefficient. By adding the prefix in- to the adjective efficient, we deny the adjective. If we call a person inefficient, we are saying the person is "not efficient", not able to finish tasks in a proper time and manner.
As for the other part of the word we're analyzing here, "credulous" comes from the Latin word "credere", which means "believe". A credulous person is a person who believes in things easily, without questioning or reasoning.
By putting in- and credulous together, we form "incredulous", or "not credulous". So an incredulous person is a person who does not believe easily - who is full of disbelief.