SRM (Standard Reference Method) is the standard beer-colour scale in North American brewing, running from roughly 2 (pale straw) up past 40 (essentially opaque black). Commercial labs measure it directly with a spectrophotometer, reading light absorbance through a beer sample; homebrewers instead estimate it from a planned grain bill using an equation, since almost nobody has lab equipment at home.
The Morey equation, and where it starts to lose accuracy
This site's SRM colour calculator uses the Morey equation, the version most current brewing software relies on because the older linear formula it replaced noticeably overstated colour on dark beers. Even Morey has a known limit, though — at very high MCU (malt colour units) values, common on the darkest stouts and imperial stouts, the equation's predictions become less reliable, and real finished beer often reads darker than the formula predicts on long boils or high-melanoidin mashes.
SRM tells you colour, not flavor intensity
A high SRM number signals dark colour, not necessarily heavy roast flavor — Schwarzbier reaches genuine black-beer SRM territory from a small, carefully limited roasted-grain addition while staying light-bodied and clean-tasting, proof that colour and flavor intensity are related but genuinely separable variables in a grain bill worth designing for independently.