King Becomes Two Pieces
For most of the game of checkers, you are moving pieces around the board one at a time. Each individual piece can only move in a certain direction and even "jumping" another player simply means taking the piece off of the board. However, once you "king" a piece, your opponent must then take back one of the pieces you lost during the first part of the game and put it on top of the piece that is now a "king." The dual appearance of the checkers signifies the piece is now a "king."
Movement Ability
The king can move around the board in such a way that other pieces cannot. Single checker pieces can only move one space at a time, unless they are jumping another piece. However, the king can move forwards AND backwards, which is part of the significance of getting one of your checker pieces "kinged" during the game. However, despite the king&'s ability to move freely, it still cannot move side to side, as this movement does not exist in the game of checkers.
Jumping Pieces
The king can jump from one end of a line, diagonally, to the other, and can even do so as long as just one piece is in its path. The king can "jump" this piece and continue to the end of the line. This is a marked departure from the way in which other pieces can move; the king is the only piece that can move across so many blocks at one time. However, If there is more than one piece in its path, the king cannot move across the length of the diagonal line.