Essentially, a streaming device (or streaming stick) allows you to stream (i.e. play) entertainment content – be it movies, TV shows, music, photos or sports – straight to your TV from a third-party provider, typically through a HDMI port.
Which apps you’ll gain access to of course depends on the streamer, and it’s also worth noting that support for a particular service doesn't mean a subscription is thrown in by default – if you want to watch Netflix, for instance, you’ll still have to pay for it separately (though as mentioned above, some products come with deals on certain streaming services).