"наверное" means "probably" when it's inserted as an explanation or afterthought into a passage that is grammatically complete without it (that's called parenthesis). For example:
Наверное, ты прав. = Perhaps, you are right. ("You are right." is complete without "наверное")
Ты, наверное, шутишь. = You are probably joking. ("Ты шутишь." is complete)
In most other situations, "наверное" is used to show certainty. But it often loses the last letter and turns into наверно. For example:
Я это знаю наверно. = I know this for sure.