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.