Think of how arithmetic sequences act. They start at a given number, and then they march up or down by the same distance each time. For example:
3, 7, 11, 15, 19, ...
this one marches up by 4 each time.
If this is plotted on a graph, the first point is (1,3), then (2,7), then (3,11), then (4,15) and so on. You'll notice that the slope is the same between all the points. In other words, you get a straight line! This is true for any arithmetic sequence.