A number line is a way to see the size of numbers by placing them along a line. A number line is usually horizontal with zero in the middle. As you move to the right the numbers are positive and increase. As you go to the left, the numbers also increase and get more and more negative.
In the applet above, adjust the arrow left and right to see this.
The arrows on each end of the line indicate that the line actually goes on forever in each direction. This reflects the fact that numbers go on forever. There is no 'largest number'. Whatever number you can think of, you can always find a larger one by simply adding one to it.