If you subtract a positive constant c from the input of a function, that will have the effect of shifting the graph of the original function to the right by c units. Because $\displaystyle\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ is being subtracted from the input, the graph of $\displaystyle{y}={\cos{\theta}}\usepackage{cmbright}\Huge\$ gets shifted to the right by $\displaystyle\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ units.