When a = 1, the graph of y = cos(x) is shown. Notice how it has a wave shape, going up and down between 1 and -1 on the y axis. The graph cuts the x axis when x = 90, 270 and so on.

If you change the value of a to 2, then the graph of y = 2cos(x) varies between -2 and 2. The value of a affects the amplitude of the graph.