An Acid Rest is a step done early in the mash around 95F by traditional brewers to lower the pH of the mash. This was traditionally done in very soft water locations like Pilsen that lacked minerals needed to acidify the mash. However modern brewers using most waters do not need to perform this step. Mineral additives for pH adjustment, highly modified grains, regular brewing water, and a much better understanding of water chemistry have largely eliminated the need for an acid rest.
The acid rest worked by breaking down organic phosphates in pale malt to create a weak acid. However, in some cases it can take hours in an acid rest to lower the pH to an acceptable range of around 5.2 pH. A more efficient method if you have very soft water is to add minerals, acid or buffering agents to reduce the pH to an acceptable range immediately.