'CTRL-click' is a computer term that refers to holding down the 'CTRL' (Control) key on your keyboard and then left-clicking your mouse at the same time:
It's a bit like playing the piano where you press two keys at once - one key being on your keyboard and the other on your mouse.
This action often gives a different result compared to just clicking the mouse alone. For instance, in many programs, 'CTRL-click' is used to select multiple items at once. If you 'CTRL-click' on different files or pictures, you can select multiple of them together, instead of just one at a time.
When PWM (Program Window Manager) is installed on your computer, CTRL-clicking on a window's title-bar brings up the PWM menu.
So, 'CTRL-click' is a handy shortcut on your computer for doing certain tasks more quickly or efficiently.