The Slide Sync feature allows you to show your slide deck via a web browser As you advance slides, your audience will see those same slides on their mobile device, in sync. 

To use Slide Sync, you'll start by following the instructions in Setting up Presentation Mode, but with one addition. To enable the Slide Sync feature, you'll need to visit Session Settings (within your session), then choose Enable/Disable Features and click the padlock to enable Presentation Sync / Slide Sync.