They know what they know and what they don't. They know what they can do and what they cannot do. When you meet someone who has certainty, you will see a person who others often follow. If you build into certainty others are likely to follow you. Certainty is a unique state, a frame of mind.
Certainty is different from confidence.
Confidence can occur in degrees. For example, we could have a little confidence, some confidence, a lot of confidence or a whole lot of confidence. You have it or you don't. If you believe you have a little certainty then you don't. This state either exists or it doesn't.