Castling is a special move that allows the King to jump two squares and the Rook to hop over him. It is essential for protecting your King early in the game.
Remember the "Untouched" Rule: You cannot castle if the King or the Rook has already moved, even if they moved back to their original squares!
In Short Castling (0-0), the King moves from e1 to g1. In Long Castling (0-0-0), the King moves from e1 to c1. Long castling takes longer to prepare because three pieces (Queen, Bishop, Knight) must be cleared first.
Strategically, castling also "connects" your Rooks. Once the space between them is empty, they can defend each other and dominate the back rank. Most grandmasters castle within the first 10 moves to ensure their King isn't caught in the crossfire of the center.
Be careful: You cannot castle to escape a check, and you cannot "jump" over a square controlled by an enemy piece. If your opponent is preventing you from castling, you must first block the attack or chase the attacking piece away.