It’s all about how a computer program would react to such a command.
In programming languages such as BASIC, a “WHILE” statement introduces a loop that keeps on going for as long as the condition remains fulfilled. After each loop, the program checks to see If the condition is true or false. If the condition is never broken the loop continues forever.
The husband acts like a computer program would and keeps getting a dozen eggs for as long as he’s in the store.
Since the wife only said “WHILE you’re in the store get a dozen eggs” but didn’t provide the husband with specific instructions for when to get out of the store, he goes to the store and acts like a computer i.e. keeps getting eggs a dozen at a time until the store runs out of eggs, at which point the husband promptly prints an error message and crashes.
It’s funnier if you understand it on your own from the start…