The nucleus accumbens, sometimes just called the "accumbens," is a
small cluster of neurons in the basal forebrain (pretty close to the
middle of the brain); actually, it's defined as the point at which the caudate and putamen meet. In neuroscientific terms, it's
responsible for reward (among other things); rats will happily run through
mazes and pull levers just to get a little stimulation there. The accumbens is heavily dopaminergic, which makes some biological psychiatrists think that the accumbens might be involved in schizophrenia.
In layman's terms, it's one of the brain regions that helps make you
feel good. Cocaine stimulates the accumbens, for example.
In Everythingian terms, it's the brain region that makes you shiver
happily and play the Super Mario Brothers 1 UP noise in your
head when you get C!ed.