The Rail Fence cipher creates a zigzag pattern using multiple "rails" (horizontal lines). The plaintext is written downwards diagonally on these rails, bouncing up when it reaches the bottom rail and down when it reaches the top rail, creating a fence-like pattern. The ciphertext is formed by reading each rail horizontally from left to right, top to bottom. Think of it like writing your message on a zigzag train track and then reading each track line by line. The number of rails determines the pattern's complexity and thus affects the cipher's strength.