VLT stands for Visible Light Transmission — it's the percentage of light that actually passes through the lens. A lower VLT means a darker lens that blocks more light, while a higher VLT lets more light in. This is the single most important spec for matching your goggles to the conditions you'll be skiing or riding in.
For bright, sunny days, go with a low VLT (3–18%) to cut glare and protect your eyes. For variable or partly cloudy weather, a medium VLT (19–45%) gives you solid versatility. If you're riding in overcast skies, heavy snow, or at night, you'll want a high VLT (46–90%) so you can read the terrain and see moguls or ice patches before you hit them.
The biggest mistake is assuming one lens handles every condition. If you ski in changing weather, look for goggles with swappable lenses — or own two pairs — so you're never stuck squinting or skiing blind.