What is the ETS Math Review?

It's a free PDF from ETS covering the core math concepts tested on the GRE Quantitative section: arithmetic, algebra, geometry, and data analysis. It works as a reference, but we don't recommend it for rebuilding your math foundations. It reads like a textbook and doesn't have enough practice problems. For that, we recommend a practice-focused book such as Manhattan's GMAT Foundations of Math. Even though it says GMAT in the title, it covers the same foundational material the GRE requires.