Body-mapped construction means the base layer uses different fabric weights or knit structures in different zones—typically a heavier, warmer knit over your core and kidneys, and lighter, more breathable mesh or open-knit panels under your arms and across your upper back. The idea is to put insulation where you're vulnerable to cold and ventilation where you sweat the most.
This makes a real difference on variable-output days, like when you're charging hard down a run then sitting still on a cold chairlift ride. The breathable underarm zones dump heat while you're working, and the reinforced core panels keep you from freezing when you stop. It's essentially smarter thermoregulation than a uniform fabric can offer.
Body-mapped construction is most worthwhile for performance skiers and anyone who runs hot and cold throughout the day. If you're mostly cruising groomers in mild conditions or prefer a simpler, budget-friendly option, a standard uniform base layer will still do the job fine.