Variation
Variation is how much things differ when you measure the same thing more than once.
Think of it this way
Scoop sugar with a spoon three times. Each scoop looks full, but they hold slightly different amounts. Small differences like that are natural. No two scoops are ever exactly the same.
Explaining variation by grade level
Ride the same bus route five days in a row. Each trip takes a different amount of time. Some days are faster. Some days are slower. That spread in travel times is variation. It shows up whenever you measure the same thing more than once.