"The Clap" is a common name for a sexually transmitted infection called Gonorrhea. Gonorrhea is caused by a bacteria and you get it by having vaginal, anal or oral sex without using a condom.
A symptom is a sign of something, usually not good. For instance, a fever, runny nose, and sore throat may be a symptom of the flu. A loud knock may be a symptom that your engine has problems....