A full shell helmet covers your entire head including your lower jaw, offering the most protection you can get on the slopes. It's built for competitive ski racing and speed disciplines where the risk of high-speed impacts is real and the consequences are severe.
However, for regular resort skiing, a full shell helmet is usually overkill. It's noticeably heavier, less ventilated, and restricts your peripheral vision — which actually makes it harder to see other skiers around you. You'll also find it less comfortable for a full day on the mountain.
Most skiers are better served by a half shell (alpine) helmet, which still provides excellent impact protection for the top and sides of your head while being lighter, more breathable, and goggle-friendly. Save the full shell for race day unless you're actually competing in speed events.