The meteors represent other devices that are logged into the same Wi-Fi network as your device. These other devices can be smartphones, tablets, PCs, Macs, game consoles, smart TVs, for example.
The users of these other devices in the same Wi-Fi network can possibly intercept your data traffic or assume your online identity under certain circumstances without protection.
Of course not every device in the same network is always a threat. The purpose of this visualization is to clarify that surfing unprotected, especially in public Wi-Fi networks, is potentially posing the risk of other people intercepting your data. Whether that is possible and how easy it is depends on multiple factors the user does not always know about when connecting to an unknown Wi-Fi network, like the manufacturer and age of the router, correct and complete usage of SSL encryption on visited websites etc.
When you activate the Online Shield for iOS by swiping the shield up, you will see that the potential threats are still there, but they will be repelled by the protective shield and "burn up". This is meant to clarify that the threats are still using the same network as you, but cannot intercept your data anymore due to the activated encryption of the Online Shield.