Client-side validation is a process that checks user input in a web form before it is sent to the server. This validation occurs in the user's browser, using languages like JavaScript. It helps ensure that the data entered meets specific criteria, such as correct formatting or required fields, which can enhance user experience by providing immediate feedback. By catching errors early, client-side validation reduces the number of invalid submissions sent to the server, saving time and resources. However, it is important to note that it should not replace server-side validation, which is necessary for security and data integrity.