If someone is skeptical, they don't believe something is true or they have doubts about it.
Being skeptical can be a good thing because it means you think carefully about things before deciding if they are true or not.
Examples:
She was skeptical about the plan.
I am skeptical about the new idea.
He is always skeptical about things he reads online.
The cat looked skeptical when I told him I could speak cat language.
I am so skeptical, I don't even believe the sky is blue!
The dog gave a skeptical look when I told him I could speak dog language.