The Polybius Square is a simple encryption technique used to encode letters into pairs of numbers. It consists of a 5x5 grid filled with the letters of the alphabet, where each letter corresponds to a unique pair of coordinates. Traditionally, the letters I and J are combined to fit the 25 spaces available in the grid. To encrypt a message, each letter is replaced by its corresponding pair of numbers. For example, if the letter A is in the first row and first column, it would be represented as "11." This method allows for straightforward encoding and decoding, making it a popular choice in historical cryptography.