Notice how the arrows on the mapping diagrams are paired with the points on the graph of the functions.
The arrow on the point on the mapping diagram target axis indicates whether the value of the function $f(x)$ is increasing (pointing up) or decreasing (pointing down).
You can move the point for $x$ on the mapping diagram to see how the function value for the function $ f(x) = \cos(x + \frac {\pi}2)$ changes both on the diagram and on the graph.
The arrow on the point on the mapping diagram target axis indicates whether the value of the function $f(x)$ is increasing (pointing up) or decreasing (pointing down).

Notice how the graph and mapping diagram visualize the fact that for the cosine function,  $\cos(x+C)$  has the same basic shape as $\cos(x)$ though it is "shifted".
See Subsection TRIG.LCOMP

Check the box to show points on the graph and arrows on the diagram for the function $ f(x) = \cos(x + \frac {\pi}2)$.
Move the slider to change the function to $\sin$ or $\tan$.