It's a $35 plug in for an HDMI port on your TV (which should be an HD TV) - you can buy through Amazon - plug it into an open HDMI port (socket) and load the Chromcast app on your phone or tablet - Android or Apple. Click on the app and it will look for your wifi connection and find your video signal (this varies according to your personal set up). Once your TV and your iPad, iPhone, etc. will show up as a connection - anything you can see on your iPad etc. can now be "cast" onto your big screen TV - the technology is still improving - you can easily see Netflix and Hulu - if u have accounts (subscriptions) - and with some ingenuity, you can cast your web browser onto your TV. Apple's Air Play is a lot easier to use - but you'll need an Apple account.
It's a thing that plugs into the back of your TV. You connect it to the home router and can send Youtube, Netflix, and Hulu videos to the TV from your phone, tablet, or laptop. You can also send your screen to the TV so people can watch what you're doing on your laptop. It's pretty awesome.