Server-Side Validation is a process where data submitted by users is checked for accuracy and security on the server before being processed. This ensures that the information meets specific criteria, such as format and completeness, helping to prevent errors and malicious attacks. By validating data on the server, applications can protect themselves from harmful inputs that could compromise security. Unlike Client-Side Validation, which occurs in the user's browser, server-side validation is more reliable because it is not dependent on the user's environment and can enforce stricter rules for data integrity.