Bubble lets you customize how your app looks on social networks when users share a link to your app.
Popular social networks let you customize how links to your website look, when posted on their platforms. You can control this setting (open graph meta tags) in the Metatags section. You can customize the image and the content of the text that will be displayed.
Here is what these settings look like for the bubble.io site.
And how this looks on Facebook.
Note that defining these settings in the Settings tab will be applied to all pages of the application. Bubble also lets you modify these values at the page level. This is in particular useful if you want to use some dynamic information (for instance, the current page's product's image) instead of a generic image. If you have some settings at the page level, they will prevail over the application's settings.