It's similar to when you wanna open a file, but doesn't know what app to open that file type with. The OS shows you a list of apps that can handle that file typ

It's similar to when you wanna open a file, but doesn't know what app to open that file type with. The OS shows you a list of apps that can handle that file type and you can choose a default one. Except this is now handled by the browser instead of the OS. And it let's the website choose which "file type", and it's not just for files but for any links. The "file type" is what the web intent is and the app can be any webapp or browser extension. Sharing in just one of the many examples of intents. You could also have one intent for editing a picture, which would open an app for editing. Or playing a video. Adding an event to the calendar etc. You can make up your own intents, it can be virtually anything. If you used android, it works just like android intents. When an android app needs to play a video, it doesn't need to say "open app X", it just needs to say "I wanna play this video" and the OS will show the user with a list of apps which registered to be able to do that.