When you say something more than once, especially to make yourself more clear, that's reiteration. If your memory for math is terrible, even your teacher's reiteration of formulas might not help you pass your quiz.
When you deliberately repeat something, you reiterate it, and the process of repeating something is reiteration. Most people expect to hear reiteration of a political candidate's campaign messages—in fact, she may give the same speech over and over again. Reiteration doesn't necessarily mean repeated words; it can instead involve something like subjects: "The artist's reiteration of themes of loss can be seen in all her work." The Latin root, reiterationem, means "repetition."