Serotonin is a neurotransmitter (a chemical messenger). It is produced by nerve cells in the brain and is used by nerves to communicate with one another. A nerve releases the serotonin that it has produced into the space surrounding it. The serotonin either travels across that space and attaches to receptors on the surface of nearby nerves or it attaches to receptors on the surface of the nerve that produced it, to be taken up by the nerve, recycled, and released again. This process is referred to as reuptake. A balance is reached for serotonin between attachment to the nearby nerves and reuptake. A medication that acts as a selective serotonin reuptake inhibitor (SSRI) blocks the reuptake of serotonin and thereby changes the level of serotonin in the brain.
Reuptake is sometimes written as re-uptake.