A Cross-Site Request Forgery (CSRF) token is a security mechanism used to defend against Cross-Site Request Forgery (CSRF) attacks. It's a randomly generated token that is included as part of a web form in the form of a hidden field or as part of a request to the server. This token is used to verify the authenticity of a request and ensure that the request comes from a legitimate user and not from an attacker.

Here's how a CSRF token works:

1. When a user logs in or creates an account on a website, they are issued a CSRF token. This token is typically valid only for the current session or a limited time.

2. The CSRF token is stored on the server and associated with the user's account or session.

3. Every time the user performs an action that requires a request to the server, the CSRF token is included in the request, typically in the form of a hidden form field.

4. The server checks whether the CSRF token in the request matches the token stored on the server. If the tokens do not match or are missing, the request is rejected as invalid, as it may have originated from an attacker.

5. If the CSRF token is correct, the request is accepted as legitimate, and the action is executed.

By using CSRF tokens, it ensures that only authorized user actions are accepted, as an attacker typically does not have access to another user's CSRF token. This significantly complicates the ability of attackers to successfully carry out CSRF attacks.

Website developers should always implement CSRF token checks in their applications, especially for actions that trigger sensitive data or actions. CSRF token checks are a best practice security mechanism and an important part of the security strategy in web application development.