It means "very sure", "absolutly" or "without a doubt". It's also used to emphasize something.
Example:
Q: Did Laura win the race?
A: Yes! She sure as hell did!
The answer "Yes, she did" would be correct, but adding "sure as hell" adds emphasis and enthusiasm to the sentence.
Q: Are you hungry?
A: No, but I could sure as hell use a drink of water!
Any phrase with the word "hell" can be offensive in some situations or with some people. You sure as hell should not use that word in a business environment. ;-)