Answer:
The graph is a velocity-time graph, showing velocity as an f(t), (function of time).
The graph depicts the following:
For 6 seconds, the mailman travels at the constant rate of 3m/s. At second 6, he abruptly stops (when he stops for the mailbox which he overshot) which is represented by the dotted vertical line. He then stays at the constant rate of 0 m/s (stationary) for 2 seconds before he goes into reverse for 2 seconds at the rate of 2m/s before arriving back at the mailbox.
Some items to note:
In a real-world situation, the abrupt stop would be unrealistic and therefore may be a slope for about 1 second.
A dotted line represents a break in the graph, for our situation, it means the velocity has dropped.
A negative velocity, like in his graph, is just going in reverse.
To find the position of the mailbox:
Change the graph to a position-time graph which shows position as an f(t). Using this new graph, you can find the position of the mailbox using time at the 10th second.
Just a quick note, changing between types of graphs is an essential part of physics and you should be able to do it with no reference. It is in your best interest to learn this to be able to apply it in the future.
Hope that helps