So far the number line started at 0 and grew to the right. But it doesn't have to stop at zero — it keeps going to the left into the negative numbers: -1, -2, -3, \dots

Think of a thermometer. Above zero it is warm; below zero it gets colder and colder. Or think of money: 5 means five in your pocket, while -5 means five that you owe. The minus sign tells you which side of zero you are on.

Going left is just subtraction that keeps going past zero. The further left you travel, the smaller the number — so -3 < -1, even though 3 > 1. A bigger debt is a smaller amount of money!

Khan Academy introduces negative numbers here: