Skepsis is when you have doubts about something or you don't believe it.
Skepsis can be good if it makes you think carefully, but too much skepsis can make you unhappy.
Examples:
Her skepsis about the plan made her ask a lot of questions.
I have a lot of skepsis about the story he told me.
His constant skepsis makes it hard for him to trust anyone.
The dog's skepsis about the new food was clear from the way he sniffed it.
Her skepsis about the magic trick made her think the magician was using a trick box.
The scientist's skepsis led her to discover the mistake in the experiment.