A decongestant is a medication used to relieve nasal or chest congestion. People most often have nasal or chest congestion due to the common cold, flu or allergies.
Technically speaking, decongestants constrict the blood flow in the area of congestion. That then decreases swelling and clears the congested area, usually nasal passages.
This is why decongestants help clear out congestion and allow you to breathe more easily.