^^^ That's not strictly true. Roku is a
device (or, more strictly, one of a range of devices) that's similar to an Amazon Fire TV Stick.
https://www.roku.com/en-gb/products/players
(Netflix, on the other hand, is a streaming service that can be accessed via such devices or, of course, directly with a smart TV).
If you've got a 'non-smart' TV, then adding a Roku device to it will make it 'smart', providing internet functionality and access to both free streaming services (such as BBC iPlayer, ITVX and YouTube), as well as to paid services (such as Netflix and Amazon Prime Video) if you pay to subscribe.
If your TV set is already 'smart' though, you might find that adding a Roku device to it will gain you very little, as many/most of the apps on the device will already be on your TV set.
Roku does have its own streaming channel though, offering some free content which isn't available on most smart TVs (with recent Hisense models being the exception to that):
https://www.roku.com/en-gb/whats-on/the-roku-channel
To see what's available via a Roku device (in order that you can compare the content with that available already available through your smart TV, Sky Q box, or whatever), go here:
https://channelstore.roku.com/en-gb/browse
(Note that the link defaults to 'Featured' channels only. You need to click through the links at the top of the page to see everything that's available).