To simplify, most nerve cells (which are the ones that transmit information from sensory organs to the brain and info from the brain out to the peripheral nervous system) have a cell body with dendrites, long thin projections from the cell body, and an axon, one long thin strip that transmits an electrochemical impulse from the dendrites along the cell body down the axon. The axon is designed to speed up this electrochemical transmission.

Between each axon and the next cell body is a gap, called the synapse. Neurotransmitters are released from the tip of the axon into the synapse, where they are taken into the cell body of the next nerve cell to continue information transmission.

When your neurotransmitter levels are off, it can affect your behavior, and serotonin has been identified as a neurotransmitter that affects depression. Sometimes your body will break down the serotonin it releases too quickly, or it will not produce enough serotonin. So this drug prevents the reuptake of serotnin by occupying the pathways that take serotonin back into a cell to be broken down and reused again. This helps to boost the natural levels of serotonin in your body.