Mahalanobis distance

Mahalanobis distance is like a measure of how far away one thing is from another. Imagine you have two friends who live far away from each other. Mahalanobis distance is a way of telling how far apart the two friends live from each other. It's kind of like drawing a straight line on a map between the two friends and measuring how long that line is. But Mahalanobis distance also takes into account things like what roads are between the two friends (like if there's a highway or just a dirt road) and how hilly or flat the land is between them. This way, we can get an idea of how hard it would be to travel between the two friends and how far apart they really are.