Just like the waterproof rating measures how effective a The North Face Denali Fleece Jacket Blue is at keeping water outside, the breathability rating of a jacket measures how effective a jacket is at transferring moisture from inside to the outside. The same fabric pores that help prevent water from penetrating inside a Moncler Clairy Down Jacket in Red, allow sweat molecules to escape and ultimately keep your warmer.
Breathability rating is measured and indicated in grams (g). The measurement is determined by finding the Moisture Vapor Transmission Rate (MVTR). The MVTR determines how many grams of sweat per 1 square meter can escape a jacket in a 24 hour period. The higher the number, the more moisture escapes and the more breathable it is. Entry-level breathable fabrics will have MVTR ratings in the range of 2,000-3000g. Fabrics at the high end of the breathability scale will have an MVTR around 25,000g.
