A ding is the sound that a bell makes — a kind of metallic, musical ringing. When you hear the ding of the doorbell, you'll know that the pizza delivery guy is here.
Bells, telephones, and doorbells all make a ding, and the word is a verb as well: "I'm going to run when I hear the dinner bell ding — I want to be first in line." Another kind of ding is a small dent or nick in a surface, the kind you might get in your car's bumper during a minor fender bender. Ding falls in the category of imitative words, which sound like their meaning.