Penfield's 3 dimensional homunculus is a physical representation of the human body, located within the brain.
The cortical homunculus represents the importance of various parts of your body as seen by your brain. There is little need for the brain to know what's going on in the arms and legs. All these limbs need to do is stay out of an open flame and get your hands and feet to the right places. The hands, the tongue, the genitals, and the facial features are extremely important, and give people a ton of sensory information. As a result, they take up a lot of brain space.
This obviously explains the symptoms that patients experience after hand injuries and emphasise the need for restoring the hand function to normal.