Imagine the type a = {name: String, age : Int}. If you want to access the age part of this type or modify only the age this is simple you would use Elm syntax like x.age or {x | age = 7 } to ease the process.

Now imagine that the a is much much more complex, like a record whose fields are also records and so on for 10 levels. Elm does not have buitin syntax to ease this case. That's where lenses comes in.

A lens is like zooming on a part of a type, enabling to access or modify this part with ease. It is strongly related to Huet's zipper.

Take the a lens of type Lens a b. It enable to access the a part of b and modify it simply. The benefits of lenses is they compose naturally.