To reiterate something is to say or do something again, or many times. Let me reiterate: if you repeat yourself, you're reiterating the thing you originally said.
This verb is from Middle English reiteraten, from Latin reiteratus, from reiterare "to repeat," from the prefix re- "again" plus iterare "to repeat, iterate." The English word iterate has the same meaning as reiterate, although it is not as commonly used.