Imagine you are a manager planning a big football event in a stadium.
The tickets are fully booked, but you want to know whether there are any free seats left.

The audience comes in a group of 3 persons (for example, father, mother, kid).
And of course, the attendees want to sit next to the persons they are coming with.

How many seats remain free, if each group of attendees consists of 3 persons, and the tickets are fully booked?

This is exactly what the modulo operation delivers:

Number of seats modulo group size = number of seats that remain unoccupied.