The definition of DIFF% (Percentage Points Difference), according to the glossary of NBA stats:
The difference between the normal percentage of a shooter on shots throughout the season and the percentage on shots when the defensive player or team is guarding the shooter. A good defensive number will be negative because the defensive player holds their opponent to a lower percentage than normal
Formula: FG% - DFG%
This is the one thing Draymond focused on when they were discussing Wemby as a prospect before the draft. He repeatedly said Wemby will enter the NBA as an elite rim protector, that will be his baseline.