Columnar transposition is a method of encrypting messages by rearranging the letters based on a specific column order. First, the plaintext is written into a grid with a fixed number of columns. The columns are then reordered according to a predetermined keyword, where each letter corresponds to a column's position. To decrypt the message, the recipient must know the keyword to reconstruct the original column order. By reading the letters vertically from the reordered columns, the original message can be revealed. This technique is simple yet effective for obscuring information from unauthorized readers.