Explanation
This is a mnemonic to help you remember the months that have 31 days. Take the first letter of each word: John Made Mary Jump Around Our Dog → J-M-M-J-A-O-D, standing for January, March, May, July, August, October, December.
Also see: Knuckles and Valleys, a visual way to remember the length of every month, not just the 31-day ones.