IPTV (Internet Protocol Television) isn't a specific provider of programming. It's a way of receiving it. (i.e. over the internet). So Amazon Fire TV sticks and Roku devices, for example, use IPTV to deliver programming to people.
However when many people refer to 'IPTV', they're actually talking about
illegal IPTV services where a device that is, in itself, perfectly legal has software added to it to enable it to illegally receive 'subscription only' services (such as Sky programming) without charge. The most common way of doing it is to install software called Kodi onto something like an Amazon Fire TV Stick. That fist step is entirely legal, since Kodi (in it's 'fresh out of the box' state) doesn't enable a user to obtain 'subscription only' programming for free. However there's then a second step, which is most definitely illegal, of adding on and using additional software in order to get Kodi to get programming, that ought to be paid for, entirely free.
This link, from Trading Standards Scotland, explains more about the illegal side of IPTV:
https://www.tsscot.co.uk/priority-areas/iptv-illicit-streaming/
So, if your post is referring to things like Amazon Fire TV Sticks or Roku devices, your answer is that they'll firstly turn a 'non-smart' TV into a 'smart' one, giving you online services, such as BBC iPlayer and other catch-up channels, plus services like YouTube free of charge. You'll then have the option to pay extra to receive things like Netflix or Amazon's video streaming service.
However if your post is referring to the 'dodgy' side of IPTV, I suggest staying on the right side of the law and keeping well clear of it!