All scope lenses are made from glass as it provides the clearest optical image. However, any time light strikes a glass surface, a certain amount on the light is reflected away. The reflection reduces the amount of light passing through the lens, which impacts the brightness.
Lens coating refers to the microscopic layer of chemical coating that is applied to the air to glass surfaces on the scope. Coating the lens reduces the glare and reduces the loss of light due to reflection. Many scope manufacturers use a magnesium fluoride film for coating. A German based scope maker called Zeiss (which is still an elite scope brand today) was the first to start using magnesium fluoride coated lens in 1935.
Generally, more coatings leads to better light transmission and better contrast. The coatings are expensive and they greatly vary in type, number, and quality.
Here are some explanations of common terms used to quantify coating:
Coated: Has a single layer of coating on at least one lens surface.
Fully Coated: Has a single layer on all air to glass surfaces.
Multicoated: Has more than one layer of coating on at least one lens surface.
Fully Multicoated: Has multiple layers of coating on all air to glass surfaces.
Here's a idea of what coated lens versus uncoated lens would like compared:
Notice the clearer and brighter difference on the coated side?
When scope shopping, take the time to read the fine print and determine how much coating the lens actually has. If you can afford it, go with fully multicoated as this is the best choice in optical quality. As mentioned before, coating is expensive and is generally reflected in the price. Fully multi-coated scopes tend to be expensive. However, if you are ever hunting or shooting in low light conditions like dawn or dusk, you will appreciate the benefits of multi-coating.