Fortnite saves its game replays in a proprietary .replay file format. These replays aren't your typical screen recordings from the player's perspective; they're a full, mostly faithful recreation of events. You can view gameplay from any angle or any other player's perspective, which helps when you're trying to figure out what you just did wrong in-game that got yourself killed.
Unfortunately, you can only view these replay files from within Fortnite itself, and the game invalidates older replays in order to ensure compatability with newer versions of the game. The expired files aren't always truly unwatchable, though. You can un-expire old replays by flipping a few bytes in the files themselves and trick the game into thinking they're still perfectly valid.