Different ways exist to start a slide show. Once a slide show is running, you can take control pressing keys or clicking the mouse buttons.
By default, a slide show always starts with the first slide. You advance manually through slides up to the last slide. You can change these settings.
Choose Slide Show - Slide Show to run the show.
If you want all shows to start from the current slide instead of the first slide, choose LibreOffice - PreferencesTools - Options - LibreOffice Impress - General and click Always with current page.
Click to advance to the next effect or to the next slide.
Press Esc to abort the show before the end.
Many more keys are available to control a slide show. You can also right-click to open a context menu with useful commands.
For an automatic change to the next slide, you must assign a slide transition to each slide.
Open the Slide Transition sidebar deck.
In the Advance slide area, click Automatically after, and select a time duration.
Click Apply to All Slides.
You can assign a different time for every slide to advance to the next slide. The rehearse timings feature can assist you to get the timing right.
To advance to the first slide, after all slides have been shown, you must set the slide show to repeat automatically.
Choose Slide Show - Slide Show Settings.
In the Type area, click Auto and select a pause time between shows.
You can start LibreOffice from a command prompt, followed by the parameter -show and an Impress filename. For example, to start the file filename.odp from the command prompt, enter the following command:
soffice -show filename.odp
This assumes that soffice is in the program path of your system, and that filename.odp is located in the current directory.