Seratonin is produced in multiple places in the body (not just the brain, and apparently, most of the seratonin in your body is not produced in the brain), but doesn't appear to cross the blood-brain barrier.
I'm not a doctor but I think what that means is "seratonin in your gut probably doesn't make it's way to the brain; but, the rest of the body is already awash in seratonin, so additional production in the gut can help increase those levels, which may have some sort of yet-to-be-understood action that has an impact on depression"
There is no serotonin in pills. SSRI stands for selective serotonin reuptake inhibitor. It is a chemical that modifies how certain cells reabsorb serotonin.
