2-way stretch means the fabric stretches in two directions—usually widthwise—so it pulls on easily and fits snugly around your head. For most resort skiers, this is plenty. It gives you a secure, comfortable fit under your helmet and goggles without feeling sloppy or loose.
The main difference from 4-way stretch is that you'll notice slightly less freedom when you're really cranking your head around or making expressive faces. But that trade-off comes with a benefit: 2-way stretch fabrics, especially fleece, tend to hold their shape better over many seasons of use.
If you're doing general resort skiing and want a reliable, comfortable balaclava, 2-way stretch hits the sweet spot. Go for 4-way stretch if you prioritize maximum freedom of movement or if you're between sizes and need the extra give to get a good seal.