Session Fixation is an attack that permits an attacker to hijack a valid user session. The attack explores a limitation in the way the web application manages the session ID more specifically the vulnerable web application. The attacker has to provide a legitimate Web application session ID and try to make the victims browser use it. Using the function document.cookie, the browser which executes the command becomes capable of fixing values inside of the cookie that it will use to keep a session between the client and the Web Application.”]
Source: https://owasp.org/www-community/attacks/Session_fixation