Reach measures the horizontal distance from the bottom bracket to the top of the head tube—basically, how far you'll reach forward when standing on the pedals. It's the most important sizing metric for modern mountain bikes because it determines your core riding position, far more than the traditional seat tube measurement that older sizing relied on.
Typical reach ranges by size run roughly: Small (410–425mm), Medium (435–455mm), Large (460–480mm), and XL (485–510mm). A longer reach gives you more stability at speed and room to move around on the bike, which aggressive riders love. A shorter reach feels more nimble and playful on tight trails.
Choose based on your riding style: if you ride fast, technical terrain and want confidence at speed, lean toward the longer end of your size range. If you prefer snappy handling and quick moves, a slightly shorter reach may suit you better. Just remember that stem length and handlebar width also affect your overall fit, so factor those in too.