Yes, lash serums can actually work but you should do your research to find the effective ones. According to Dr. Joshua Zeichner, director at Mount Sinai in NYC91, they work by hydrating and coating the surface of the lash to add volume. They also create a favorable environment to encourage hair follicle growth. However, most of these serums take time to take effect and require consistent application -- there's no such thing as a free lunch.
If you're looking for a good serum, the Shiseido Lash and Brown Serum30 is a tried & true favorite.