Mesh-lined vents have a thin mesh fabric backing behind the zipper opening. When you unzip the vent to cool down, the mesh lets air flow through but blocks snow from getting inside your pants. This is especially useful if you like to crack your vents open while you're still riding rather than stopping to adjust.
The main advantage shows up on deep powder days or in wet, heavy snow conditions. Without mesh backing, open vents can funnel snow right against your base layers—which defeats the purpose of staying warm and dry. The tradeoff is that the mesh slightly reduces maximum airflow compared to wide-open unlined vents, but for most riders, the protection is worth it.
If you ride in areas with frequent powder or tend to run your vents while moving, mesh-lined vents are a smart feature to look for. They're a great all-around choice for resort and backcountry riding alike.