When you plug a Chromebook into a TV (That's what I'm assuming you're doing) it automatically makes the display "extend" off of the laptop's built-in display. If you want to watch a video on the TV, without making the video/tab full screen, click on it and drag it off the right edge of your screen. This should work to move the video onto the TV. If you want both your Chromebook and TV to show the (same) video, there is an option in settings to "duplicate" your display. However, this might make the TV's picture worse, unless you have a high-end, expensive Chromebook-- lower display resolutions make the picture look worse.