A Pseudo-Legal Move is legal in the sense that it is consistent with the current board representation it is assigned to, and it must be member of all pseudo legal generated moves for that position and side to move. For a test of a given but not already generated move, the moving piece on its valid origin square of the board must have a valid target square according to its possible move directions and distances. The occupancy of the target square in conjunction with its piece color (if any piece), or in case of distant sliding moves and double pawn pushes, the occupancy of squares between origin and target must be further considered, to determine the move is pseudo legal quiet or capture (including promotion). En passant needs the target square trigger set from previous double push and special square tests. Castling requires appropriate castling rights, occupancy- and attacking square checks.
Pseudo-legal moves may still be illegal if they leave the own king in check, most often if the king was already in check before, or the moving piece was absolutely pinned. Pseudo-legal moves versus strictly legal moves is a matter of move generation, and also playing hash- or even killer moves immediately without explicit move generation, but a pseudo legality test.