A webview is an HTML page that opens in the mobile app thanks to a webview browser which is, in most cases, a restricted version of the default phone browser (Chrome/Safari).
Code entered into the main webview interaction field is stored into a HTML file that will be hosted in the BEAR platform. You can embed in this HTML file some inline CSS or JS code thanks to the style and script tags. You can also call external resources such as a style link href=".../mystyle.css" rel="stylesheet", or a script src=".../myscript.js" though CSS or JS files won't be stored on ARGO servers.
Many possible scenarii come with webview interactions. For instance, you may show animations, quizzes, tutorials, thus deploying rich content. Have a look at that splendid animation developed by Edumedia.
Basic HTML security rules apply to webviews, including "cross-domain" or "mixed content" issues. Moreover, because webview browsers are restricted ones, some additional limits may apply: forbidden access to hardware resources (including camera/microphone/gps access), automatic media playback blocked, etc.
For more information about webviews potential and constraints, we recommend that you read the Android and iOS technical documentation:
You can also read this very good tutorial: https://developer.telerik.com/featured/what-is-a-webview/