Serotonin is a neurotransmitter - a chemical in the brain - that is associated with mood. Too little serotonin is related to depression. Serotonin is both produced and reabsorbed by the brain; many anti-depressants work by blocking the re-absorbtion.
Serotonin is a neurotransmitter. It is commonly prescribed for psychiatric or neurological problems. Without knowing more, it is impossible to tell exactly why your friend takes it.