Input validation is the process of ensuring that any data that is entered into a system is valid and meets certain criteria. This can include things like ensuring that a phone number is in the correct format, or that an email address is valid. Input validation is important for security and accuracy, as it can prevent issues like data breaches or errors in data processing. In the context of the ShortPixel Enable Media Replace plugin, input validation might include ensuring that any media files that are uploaded meet certain criteria, such as file type or size limits.