What is rote memorization and what does it have to do with mathematical facts?

Rote memorization is the act of memorizing via repetition.

There are some connections between rote memorization and mathematical facts. For one, you may have been told to memorize multiplication facts, like how 8 * 5 = 40 or 7 * 4 = 28. You may have had to look at a multiplication table repeatedly, use flashcards, etc. That is basically learning through repetition.

Furthermore, there are math formulas that you may need to remember (and deriving such formulas may be out of reach for now).

By doing many math exercises that use a certain math formula, you are essentially doing rote memorization; you are repeatedly using a formula, and over time it will be remembered.