Brix (°Bx) is a gravity scale measuring dissolved solids as a percentage of a liquid's total weight — the standard scale in winemaking, and increasingly common in cidermaking, where refractometers calibrated in Brix are the typical tool for checking juice sugar content before fermentation.
Close enough to Plato to cause real confusion
For solutions in the gravity range wort and juice typically occupy, Brix and Plato read within roughly 0.2 degrees of each other — close enough that some brewers and cidermakers treat the two as interchangeable, which is usually fine in practice but technically imprecise, since the two scales are calibrated against slightly different reference solutions. A cidermaker reading a Brix figure off a refractometer and comparing it against a beer recipe's Plato-based target is close enough for most practical purposes without needing an exact conversion.
Why this matters more for cider and mead than for beer
Because refractometers are especially common in winemaking and cidermaking (a few drops of juice give a fast reading without needing a full hydrometer sample), Brix shows up far more often on the cider and mead side of homebrewing than on the beer side, where specific gravity and a hydrometer remain the dominant convention — worth knowing if a cider recipe's sugar target is given in Brix rather than the specific gravity terms this site's own style pages use.