A number line is a straight line that represents numbers in a sequential order. It typically has arrows on both ends to indicate that numbers continue infinitely in both directions. Each point on the line corresponds to a specific number, with equal spacing between them. The center of the number line usually represents zero, with positive numbers extending to the right and negative numbers extending to the left. Number lines are useful for visualizing mathematical concepts such as addition, subtraction, and the relationship between numbers. They help in understanding how numbers compare to one another, making it easier to grasp concepts like integers, fractions, and decimals.