Acid free is a term commonly used when talking about mat boards or paper. Acid free papers are considered the best in particular for archival quality framing as they do not easily yellow or fade with age.

Acid free paper is paper with a neutral or basic pH level of PH7.0 to PH9.0 This is achieved during production. When still in pulp stage it is treated with a small trace amount of calcium carbonate or magnesium bicarbonate. This effectively neutralizes the natural acids in the wood pulp. Thus creating what's called acid free paper. This process extends the life of the paper as well as making it more easily recycled. 

Acid-free paper can often be identified by the 'infinity' symbol.