How to work with hidden sides
To make slide hidden, in the slide thumbnails, right-click on the slide and choose Hide Slide in the popup menu:
The slide will look hidden:
Note: If you use the slide numbers, be careful – for PowerPoint, it is still a slide, and you will see the third slide with number 3, not 2 if the second slide is hidden. It is recommended to put all your hidden slides to the end of your presentation.
To unhide the hidden slide, just right-click in it and click on Hide Slide in the popup menu:
See also this tip in French: Comment travailler avec des diapositives masques.