What is a Cross Site Request Forgery Attack?
The CSRF is an attack that forces an end user to perform unwanted actions and without noticing on a web application he/she is currently authenticated.
CSRF attacks specifically target requests that make modifications, not data theft, because the attacker has no way of seeing the response of the falsified request. The outcome of the actions is what interests the attacker.
This type of attack is based on the fact that when a user is authenticated on an application, it will usually provide a session ID that its browser stores in a cookie.
Each time the user sends a request to the server, the browser will also automatically send this session cookie. You can find in linked article more information about CSRF attacks.
Keep in mind that a CSRF attack only needs that the user stays connected (without having an open page or tab of the website) to be working.