Castling
Castling is the only move in chess that moves two of your pieces at once: the king goes two squares toward a rook, and the rook lands on the square the king passed over. It tucks the king away from the open central files and activates the rook in a single tempo. Neither piece may have moved before, the squares between them must be empty, and the king may not castle while in check, through an attacked square, or into check.
After O-O the white king sits on g1 and the rook on f1; Black still holds both castling rights (kq).