From the Super Mario Wiki
Highlight Reels are replays of races and battles in Mario Kart 8 and Mario Kart 8 Deluxe. After a race or battle, the player has the option to see the highlights of the race or battle. They show the player(s) in portions of the race or battle for 30 seconds. This can be edited to show highlights for 45 and 60 seconds, or the entire race or battle. The drivers it features can also be changed, as well as the types of events (action, drifting, items) and if the course music will play, or just sounds. While watching the highlight reels, the player can speed it up, slow it down, and rewind it.
Up to six Highlight Reels can be marked as favorites by pressing the button at the end of the race or battle. Highlight reels can be viewed in Mario Kart TV. Highlight Reels from around the world can be viewed in the sections Global: Popular and Global: Recent with Internet connection and information from Miiverse activity.
In Mario Kart 8, players could originally share their Highlight Reels worldwide, but as of November 7, 2017, the Miiverse service was terminated, preventing players from uploading them.