Description
Click-through means that you can click through something. Normally, the things behind it can then be clicked (or selected), while normally they wouldn't be clickable, because the element in front of it would catch the click-event [1].
CSS
To make something click-through in CSS, many use pointer-events: none;
, which means that, when an element is clicked, a pointer event (in this case, a click-event [1]) will not be fired.
Instead, you can click (or select) the elements behind it. This is basically useful for (partly or semi-)transparent images.
It is also used to simply disable click-functionality (disabled links for example). Mind though, when using pointer-events: none;
you disable all pointer events, not only the click-event (which is necessary for a click-through element); hovers and other pointer events will be ignored as well.
[1] Basically an event saying "Hey, I have been clicked!".