An iOS update is a software release from Apple that includes new features, security patches, and bug fixes for the iPhone operating system. It aims to enhance performance, provide improved security, and introduce new functionalities to maintain the stability and effectiveness of the device. iOS updates are typically versioned with numbers and letters, such as iOS 18.4.1, where the first number indicates the major version, the second number represents the minor update, and the third number signifies a specific patch or bug fix.