A dial or BOA fit system uses a small knob at the back of the helmet that tightens an internal harness around your head in small increments. Think of it like a quick-adjust belt — you spin the dial to snug things up or push it in to loosen. It lets you fine-tune your fit without taking the helmet off.
The biggest advantage is convenience and precision. You can adjust it one-handed, even with gloves on, which is clutch when you're adding or removing a beanie mid-day or dialing in comfort between runs. More importantly, it distributes pressure evenly around your head, keeping the helmet securely in place during a crash — which is the whole point of wearing one.
If you're shopping for a mid-range or premium helmet, you'll almost always get this system, and it's worth seeking out. It's the most effective way to get a proper, safe fit, especially when you're juggling goggles and layers underneath.