It's a box you plug into the tv which allows you to watch stuff on the tv. Apple TV comes preloaded with YouTube and various other pointless free things and has radio stations. It also has Netflix and now tv which you have to pay for but which are pretty good. You can also stream things through the iPhone or iPad onto the telly like channel 4 and iplayer. It's good if you don't have/want freeview and don't want to pay for extra Sky or whatever
I use it for Netflix, and also to display the iPad screen, for example if we have family round after a holiday, we show the photos on the TV big screen, so can all see them at once and chat about them.
By the look of it you already have a smart tv so there would be no need to get one if streaming and XOS device to it as your tv should allow it via your own wifi network. Most tv's have enabled apps pre-installed, like you use Netflix, Youtube etc so also no need to get one for those reasons. To be fair since the advent of smart tv's i see less reason to actually need to get one.