Defensive Rating is a basketball metric that measures the defensive productivity of a team. It is the total points a team allows per 100 possessions, the mirror image of Offensive Rating, so a lower Defensive Rating means a stingier defense.
The league average this season is 114.7. Anything below league average is a good Defensive Rating, since lower means the defense allows fewer points per 100 possessions.
Usually, yes, a lower Defensive Rating is better. It can depend somewhat on the strength of the opponents a team has actually faced, though, since a schedule full of weak offenses will flatter a defense's rating compared to a tougher slate.
Not directly. Blowout minutes, where both teams often empty the bench and play more loosely, can pull a team's Defensive Rating in either direction depending on how those stretches go, since the raw points-per-100-possessions math doesn't distinguish a competitive possession from a garbage-time one.