Queenside (long) castling is castling toward the queenside rook. The king moves two squares from e1 to c1 (or e8 to c8) and the rook jumps from a1 to d1 (a8 to d8). It's called 'long' castling because the rook travels further. The usual castling rules apply: neither piece may have moved, the squares between must be empty, and the king can't pass through check.