If you are incredulous that means you can't or won't believe something. If you tell people about those aliens you met the other night, they'll probably give you an incredulous look.
Incredulous is the opposite of credulous, which means "believing too easily." Both words come from the Latin word credere, which means "to believe." Incredulous is stronger than skeptical; if you're incredulous of something, you refuse to believe it, but if you're skeptical, you're doubtful but you haven't ruled it out completely. If someone insists that your best friend is actually an underworld spy, you'll probably look at them with incredulous anger.