Rebound Percentage is the percentage of missed shots a player grabs while on the court. It estimates the share of all available rebounds, both offensive and defensive, that a player pulls down during the minutes they play.
Roughly illustrative split, not a fixed ratio. Defensive rebounds are simply far more available per game than offensive ones league-wide, which is part of why DREB% usually outweighs OREB% within a player's total REB%.
It depends on position. Here are the position averages this season, anything above your own position's average is a reasonable bar for "good." Minutes-weighted, min. 20 GP.
Top 5 in the NBA this season, min. 20 games played.
It depends on position. Minutes-weighted averages this season: PG 7.3%, SG 7.3%, SF 9.2%, PF 11.3%, C 16.5%. Anything above the average for a player's own position is a reasonable bar for "good."
Certain players play outside the restricted area while others stay near the paint and grab more boards, which isn't directly correlated with height. Positioning, timing, and how far a player roams from the basket matter as much as size.
REB% is the combined of the two, offensive rebound percentage and defensive rebound percentage together into one overall number.