Functionalism is a theory of mind that makes it possible to imagine making minds in something else than the brain. The popular analogue (to the mind) used for explaining functionalism is a carburator. A carburator has a functional description (1), and they can have technical descriptions (2 - 6):

1. A carburator is a piece of machinery that usually sits in a combustion engine, its job is to supply the engine with fuel in adequate amounts.
2. 
3. 
4. 
5. 
6. 

All of the designs (2 - 6) are able to supply a combustion engine with adequate amounts of fuel, and therefore they fulfill the functional description. A functional theory of mind will claim that the brain is a design that fulfills the functional description of containing a mind, but that there probably are other ways to realize it.