Optics (often – and in earlier versions of Freya – referred to as lenses) are a functional technique to enable you to work with complex data structures more easily. As data structures are generally immutable, modifying a data structure (or returning a new instance with the changes reflected) can be quite onerous if the data structure is large and complex, and the area you wish to change is deep within the data structure.
Optics, as their name implies, enable you to focus on a particular part of a data structure, treating it as if it were a normal top level instance of some data. You can think of doing your work through a lens (an optic), which will handle the mechanics (or optics, rather) of data structure modification for you.
Freya uses the Aether optics library. The guides to optics found there give a good introduction to the principles, along with more general information about the library itself (which is used extensively in Freya).