It depends somewhat on whether it is a land snail or an aquatic snail. Both land and water snails are mainly herbivorous, which means they eat plant life
I have land snails in my garden and they eat the leaves of my plants. They are pests in the garden and ruin the appearance of plants.
Water snails, such as the type you might put in an aquarium, mainly feed on the algae that will naturally grow on the inside of your fish tank or fish bowl. If you have any plants in the aquarium, they can also devour those too.