I had to look it up, too:
“What is the Ding Dong ditch prank?
What is ‘ding-dong-ditch’? Dictionary.com defines “ding-dong-ditching” as: “a children’s prank involving ringing someone’s doorbell and running away before they answer.” Ding-dong ditch is commonly played by elementary school-age children.”
Yes we did this as kids and it was harmless prank back then. But people are uptight and dangerous these days so not a good idea.