A user's player data contains any player-related data on a server, like their inventory, achievements, location, statistics, ender chest etc. Sometimes it's required to delete someone's player file. For example, if they have a broken item in their inventory. Deleting someone's player file causes any data related to the player to be reset.
Delete a player file
Before deleting the player data, make sure that your server is offline. Otherwise, the server might overwrite the deleted data again when it saves. After stopping your server, go to the Players page and click on the arrow next to the respective player. After that, simply choose the type of data you want to delete and click "Delete player data."