Underarm mesh panels are small sections of breathable, perforated fabric placed right where your arms meet your torso — one of the biggest heat-trapping zones on your body while riding. They let hot air escape and cooler air flow in without turning the whole jersey into a wind tunnel.
This is the most common ventilation setup you'll find on cycling jerseys, and for good reason. It hits the sweet spot between cooling you down and keeping the rest of the jersey solid enough to block light wind and maintain coverage. You get real airflow where you sweat the most, but the front, shoulders, and pockets still function normally.
Underarm mesh is a great feature for summer jerseys and even all-around performance jerseys you'll wear from spring through early fall. If you ride in warm conditions regularly, it's absolutely worth seeking out.