Fixed vents are permanently open channels that let consistent airflow through your helmet — you can't close them with sliders or plugs. They're simpler, with fewer moving parts, which can make the helmet slightly lighter and often more affordable.
The tradeoff is flexibility. On frigid days or when you're riding fast, those always-open vents can let in more cold air than you'd like, and there's no way to dial it down. Fixed vents work best if you mostly ski in milder conditions or warm spring weather where constant airflow is actually a benefit.
For most skiers, adjustable vents are the safer bet because you can adapt to changing weather throughout the day. But if you're on a budget and primarily ski in warmer conditions, a fixed-vent helmet can still keep you comfortable — just be aware you'll have less control on cold days.