The word "sure" has multiple meanings depending on its context.
1) As an adjective, it can be used to express confidence or certainty about something. For example, "I am sure I saw a cat in my backyard." In this sentence, the speaker is confident that they did indeed see a cat in their yard.
2) It can also be used as a verb meaning to make someone more certain or convinced of something. For instance, "The evidence surely proves his guilt beyond any doubt." Here, the word "surely" means to make sure or to convince with confidence.
3) As an adverb, it is often used to express agreement or confirmation. An example sentence would be: "I am sure you're right about that." In this case, the speaker uses "sure" as a way of acknowledging and agreeing with what was said.