passive vs active rooks

I read a bit about this in a Silman book, but he doesn't define what they are. Is an active rook an attacking rook ? Passive rook a defending one? Doesn't seem that simple

Piece activity is when your pieces control a lot of squares.

Passive pieces is when they do not control a lot of squares.

The more squares a piece controls.  The more active it is.

Rooks are more active on open files for example. if your rook is sat behind a pawn, shuffling it over to an open file will improve its activity . Pushing pawns also opens up files and diagonals which will improve the activity of pieces if they are blocked. Tactics and opportunity flows from better piece activity. I'd prefer the terms active an inactive. I'd say a passive piece is not making any threats or creating tension in the game.

The opposite is also true. If your opponent has his bishop on a long open diagonal, pushing a pawn into the diagonal will reduce the activity of the bishop (assume pawn is supported so he cant just take it). Look for opportunities to limit the activity and therefore usefulness of your opponents good pieces and dont give him s reason to improve his poor pieces by attacking them or helping him trade them off.

Correction. exchanging pawns opens up files. Pushing them opens diagonals. Both can affect piece activity.

An active rook, or any material for that case, is one that conditions the opponent's (logical) choices by means of threats or by controlling key squares.