Means a quick rule of thumb. something you can use to approximate a result really quickly.
Means a quick mental shortcut.
Like if i asked you how long would it take my money to double at 8% annual interest?
My answer, in a flash would be 9 years.
How did I know that, so quickly?
Because I used a quick heuristic. The Rule of 72.
If I divide 72 by the rate of interest, I can quickly get a grasp of how quickly my money will double.