Web Content
Any web content like Youtube, Slideshare, Google Slides, etc. can be embedded in your presentation.
Add a web content
When you add a web content, you have to define a title, to identify your content.
Then, you have to include the embed code of your content. You can find this code on the website where your content is hosted. For example, on Youtube, you can click on the share button and then on the embed button to get the code.
INFO
Embed codes are usually in an iframe tag (e.g. <iframe src="https://www.youtube.com/embed/VIDEO_ID"></iframe>
).
When you check "Attendee can view the web content on their device", your audience could see the embed content during your presentation, on their device.