When a column gets too full.
Moving a value to the next column when a sum exceeds 9.
This technique allows us to add infinite numbers on paper.
Write the 'tens' digit on top of the next column.
Plain English: A column can only hold one digit (0-9). If you add 5 + 7 and get 12, the '1' has to move to the next house on the left.
Real-world example: You have 9 pennies. You find 2 more. You trade 10 pennies for a Dime. That's carrying.
Forgetting to add the little '1' you carried over!