YouTube has put an Easter egg into its videos - the classic game Snake.
To play the game on YouTube, you must be watching a video in the new player, not the old one. Start the video, then pause it. Hold down the left arrow key and a string of dots will move across the image. This is the snake. There will be a flashing dot in the field of play. You must guide the snake (using the arrow keys) to “eat” this dot. If you do, the snake grows in length one dot. You must then guide the snake to eat another dot. The object of the game is to eat as many dots as possible and grow your snake as long as possible. You must avoid the sides of the playing field, and if your snake grows a sufficient length, you must also keep from running into your own tail.
To start a new game, hold down the left arrow key. This also works in full screen, if you’re having difficulty playing in the default screen size.
The new player is part of YouTube’s site. It’s not something you need to “get” as it is a native part of YouTube, independent of your browser or OS. You just need to find a video that is playing on the new player instead of the old. At first, YouTube has been rolling the new player out on videos without ads. They plan to eventually have all videos play in the new player. You just have to search until you find one that does. As for why they’re doing it, it’s Google. They just like to do cool things, like when they had Pac-Man playable in their logo recently.