Cross-Site Request Forgery (CSRF) is a security vector in which an unauthorized request is accepted by a server on behalf of another user; it is essentially an exploit of the trust a site places on a user's browser.
The typical mitigation is to create a one-time token that is transmitted as part of the original form, and which must then be transmitted back by the client. This token expires after first submission or after a short amount of time, preventing replays or further submissions. If the token provided does not match what was originally sent, an error should be returned.
mezzio-csrf provides utilities for both generating CSRF tokens, as well as validating them. Tokens are stored within a session, and expire after any attempt to validate.