Client-side validation is a check performed in the browser before a form or request is submitted. It can improve usability by catching obvious mistakes early, such as missing fields, bad formats, or values that exceed a limit. But it is not a security control, because the browser belongs to the user and can be modified, bypassed, or ignored entirely.
In cyber security, this matters because attackers can send requests directly to the backend, change hidden parameters, or use automation to skip the interface. If the server trusts a browser-only rule, a booking limit, price check, or account restriction can be defeated. Defenders should treat client-side validation as convenience only and enforce the same rule on the server, along with strong authorization and input validation. In practice, secure systems re-check every sensitive action on the backend, because the browser is never the final boundary.