See allows you to share the contents of your mobile screen or desktop with your friends while you chat with them.
On the iPhone and Android your friends can see your screen or you but not both at the same time. So when you are sharing your screen then your friends see your screen and hear you.
On the web you can choose however many apps you want to share or your whole desktop while everyone can see your video.
Your friends see everything on your screen including any messages that popup so you may want to put your device on Do Not Disturb mode to prevent any messages appear.