An Xtream IPTV subscription is access to television delivered over the internet instead of cable or satellite, usually managed through the Xtream Codes panel format that became the industry standard for IPTV resellers. When I set my first one up, the provider sent a server URL, username, and password, which I entered into an app like Tivimate; the playlist and guide loaded within seconds.
It is the same idea behind Internet Protocol television: live channels and on-demand video streamed over IP networks rather than broadcast spectrum. The Xtream part just refers to the login API format apps use to fetch streams and EPG data, which is why so many players support it natively.
One caveat: the original Xtream Codes platform was raided in 2019, which TorrentFreak covered when pirate IPTV traffic dropped sharply afterward. Today's providers mostly run clones of that software, so quality varies wildly between sellers.
For big one-off events I prefer the official route, like ordering a PPV event through DirecTV, since the stream is guaranteed and fully legal. My rule: trial any subscription before paying for a year, and test catch-up and VOD during the trial.