When a = 1 and b= 1, the graph of y = cos(x) is shown. It has the familiar 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 keep a = 1 and change the value of b to 2, then the graph of y = cos(2x) still varies between -1 and 1, but it crosses the x axis more frequently (at x = 45, 135, 225, 315 and so on). The value of b affects the frequency of the graph.