A number line is a line (or line segment or ray) that represents all real numbers. Each point on a number line corresponds to a real number. Number lines quickly enable us to see the relative magnitude of the values on the number line, with numbers to the left being smaller than numbers on the right.
Above is an example of a number line. The number line propagates infinitely in both directions starting from 0. A number line can also be a line segment (has two endpoints)
or a ray (extends infinitely in one direction).
Number lines can be useful for learning or teaching counting and addition. They can also be used to introduce the concept of negative numbers. Each tick mark on a number line indicates the same interval between each number (typically 1, as in all the examples above). This is useful for visualizing negative numbers as the "distance" away from 0 in the negative direction on a number line.
It can also help with the concept of subtracting negative numbers. The concept of subtracting negative numbers may be a bit confusing at first, since subtraction is often associated with resulting in a smaller value (since this is true with positive numbers), but a number line can help us see why subtracting a negative number results in a larger number.
Subtract -3 from 4 using a number line.
3 - (-4) = 3 + 4 = 7
-3 is 3 spaces from 0 in the negative direction, and 4 is 4 spaces from 0, so they must be 3 + 4 spaces from each other, or 7. We could also have just counted the total number of spaces between the two along the number line.