Google Stadia is a streaming service for playing video games over the internet. Instead of having to buy a high-spec computer or console to play games, people need only a Wi-Fi controller and a dongle for their TV. The games run remotely on servers in data centres. As a player moves around in a game, video of the gameplay will be streamed to their device.
Providing the internet connection is good enough, this will happen seamlessly. Players can also switch devices mid-game, such as by starting their game in a web browser on a laptop and then changing to a mobile phone.