Using a percentage allows you to only apply rules to a subset of users. This is useful if you want to roll out a feature to a small percentage of users first and then increase it over time.
Adding a percentage to a flag can be done in the top half of the flag editor.
The percentage is applied to the flag before the rules are evaluated. If the percentage is 80%, the flag will be evaluated for 80% of the users and disabled for the other 20%.
Set the percentage to 0 or click the small trash icon to remove the percentage.
If no percentage is set, the flag will be evaluated for all users.