Player files contain any player-related data on a server. This includes inventories, achievements, location, etc.
Sometimes it's required to delete someone's player file. Usually if a player got bugged or if someone did some experiments with a player like teleporting him to invalid coordinates.
Deleting someone's player file causes any data related to the player to be reset.
Delete a player file
After stopping your server, go to the Worlds page and open your world files.
Navigate to the "playerdata" directory.
This folder contains all playerdata. Each player on your server has their own file. Find the file that is named after the UUID of the player. You can see the username a file belongs to next to its filename.
To delete a file, click the trashcan icon.