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 length, and helps prevent malicious data from entering the system. By validating data on the server, developers can protect their applications from common vulnerabilities, such as SQL injection and cross-site scripting. This type of validation is crucial for maintaining data integrity and ensuring that only valid information is stored in the database.