Just to add to Sunk's post:
If your TV is already 'smart' (i.e. it can connect to the internet) you might have no use for an Amazon Fire TV stick, a Now TV stick, a Roku Streaming Stick or whatever (although they can still add extra functionality to 'smart' TVs, depending upon exactly what those TVs can already do without one, which varies across different TV models).
If your TV isn't 'smart', then plugging one of the above devices into an HDMI port on your telly will make it so, meaning that it can take signals from your home wi-fi router. You can then use the free apps on the device to watch catch-up TV services, such as BBC iPlayer ITV Hub, All 4, My 5 and UKTV Play. You can also use other free apps to access things like Youtube on your telly, instead of on your computer. Further, if you so choose, you can use subscription-based apps to watch services like Netflix and Disney+. (If you choose a Now TV stick you can subscribe to packages that give you access, via your router, to certain services that you'd otherwise need a Sky TV subscription to access).