Rationalism is a doctrine that prioritizes reason over emotion. If you're scared of the dark and convinced there's a monster in your closet, your parents will beg you to be rational, and rely on what you know (empty closet) instead of what your fear (monster!).
French philosopher René Descartes, who wrote "I think therefore I am," is considered the father of rationalism. He believed that eternal truths can only be discovered and tested through reason. The "scientific method" is one of rationalism's great gifts to mankind––if something cannot be proven through a repeatable experiment, you do not consider it to be true.