A number line represents numbers in order on a line, showing their sequence and spacing. Number lines help visualize addition and subtraction as movements and support skip-counting.
You already know how to count from 0 all the way to 20 (your prerequisite). When you count — 1, 2, 3, 4... — you're already moving through numbers in order. The number line makes that movement visible by putting every number at a fixed spot on a straight line, equally spaced from its neighbors. Zero is all the way to the left. Twenty is all the way to the right. Every number in between has its own home, and the numbers are always in the same order, equally far apart.
The spacing matters. On a number line, the distance from 3 to 7 is exactly the same as the distance from 10 to 14 — it's always four steps. This is different from just knowing what comes next when you count. The number line shows you how far apart numbers are, not just what order they go in. You can look at a number line and see at a glance that 15 is much farther from 2 than it is from 12.
Here is the most powerful thing about the number line: addition means moving to the right and subtraction means moving to the left. If you start at 5 and add 3, you take 3 hops to the right and land on 8. If you start at 12 and subtract 4, you take 4 hops to the left and land on 8. You can put your finger on a number and physically hop to the answer — the number line turns arithmetic into movement that you can see and touch.
The number line also helps you see patterns. If you skip-count by 2s starting from 0, you land on 0, 2, 4, 6, 8... — always landing on even numbers, equally spaced, forming a regular pattern. The number line is one of the most useful tools in all of mathematics, and you'll keep using it — with bigger numbers, with fractions, and eventually with negative numbers — for years to come.