Widgets are essentially mini applications you can run on your website. This could be song from SoundCloud, the review of your Restaurant on TripAdvisor, a Google Calendar displaying your events or perhaps a web forum or live chat app.
Lots of modern websites offer ways to embed their content on social media or websites, they typically provide a bit of HTML code, well, any site or service that does this you can embed on a Website Builder website. Simply click Add in the side bar, click Widgets and click Embed HTML. Now you will be able to place a box on your site where you want your embed to be seen, once you have placed the box simply click Edit HTML and paste in the code you obtained from the other website.
Other popular embedded services include Spotify, Twitter Feeds, Vimeo videos and many more. If there is HTML for it, you can use it on Website Builder!