Depending on the context, all things apply.
There are things you know, usually simple and self-evident like “I know were my glasses are, because I am wearing them”.
There are things you believe, usually things that are abstract and vague and usually hopeful like “I believe that next year I will manage to achieve this or that goal”.
There are things you (should) question, usually most of reality and whatever happens daily.
There are things you wonder, usually things about your specific interests or long-term goals (e.g. what is happiness? the meaning of life etc).