Every day, glands in the lining of your nose and throat produce about 1 to 2 quarts of mucus -- a wet, gooey substance that moistens your nasal passages and helps trap and destroy foreign invaders like bacteria and viruses before they can get into your body and cause infection. Normally, you don't notice all that mucus because it drips harmlessly down the back of your throat and you swallow it.
Only when your body produces more thin mucus than usual, or the mucus is thicker than normal, does it become more noticeable. Excess mucus can come out the front of the nose in the form of a runny nose. When the mucus runs down the back of the nose to the throat, it's called postnasal drip.