Selective serotonin reuptake inhibitor
Many researchers think low levels of serotonin in the brain can help cause depression.  If a person's brain does not have enough serotonin, the serotonin cannot do its job of controlling their mood.  This can make the person depressed.  (It can also cause other symptoms of depression, like not having any appetite, not being able to sleep, or sleeping too much - because serotonin controls appetite and sleep too.)