When you are using statistics to understand a relationship between two things, you have to be careful that a third thing doesn't cause a problem.
Suppose you want to see how human height is caused racial genetics. You measure the height of 100 randomly selected white people from around the world, and 100 randomly selected black people from around the world. You find that white people are taller on average. But then you "control for" diet by comparing the white people who ate very little with the black people who ate very little, and comparing the white people who ate a lot with the black people who ate a lot, and you discover that race has very little influence on height. It turns out most white people are from rich countries where people eat a lot and most black people are from Africa where people don't have as much food.
When doing statistical studies, you have to control for things that might mess up your results.