A ping is a sharp, high-pitched, somewhat musical sound. Quickly tapping a spoon against a crystal glass makes a ping. If you ping someone, you contact them via computer or phone, which might ping when the message comes through.
The word ping is onomatopoeic, or imitative — in other words, it sounds just like its meaning. Things that might ping include your electronic alarm clock, some video games, a stone against your car's windshield, and the timer on your microwave. In the 1800s, a ping was primarily the sound a bullet made as it hit something — today it's just as likely to mean "a message sent from one computer to another."