Reach measures the horizontal distance from the center of the bottom bracket to the top of the head tube—basically, it tells you how stretched out you'll be on the bike. A longer reach puts you in a lower, more aerodynamic position that's ideal for racing and fast group rides. A shorter reach gives you a more compact, upright position that's better for endurance riding and all-day comfort.
Reach is harder to adjust after the fact than stack height. You can tweak it with a different stem, but going more than 10mm longer or shorter than stock usually hurts the bike's handling. That's why it's important to compare reach numbers across bikes in the same size to understand their true fit character.
Don't confuse reach with top tube length—reach is the more reliable measurement because it accounts for seat tube angle. Also keep in mind that your saddle setback is part of your overall reach to the handlebars, so consider it when dialing in your fit.