This app is about visualizing ten’s complements.
About ten's complements:
The "ten's complement" is the number that you must add to get 10. For example, the ten's complement of 6 is 4. And the ten's complement of 3 is 7.
Having a clear visual image of single-digit numbers and their ten's complements is known to be very helpful in doing addition and subtraction. It is also a key step in facilitating mental math.