What's purpose of t_cookie_string variable?

[modest]

So I have been using Mantis for over two months now and I am starting to tinker with some Mantis code, specifically the login/authentication code. I noticed that Mantis authentication file (authentication_api.php) uses a lot of reference to $t_cookie_string variable. Can anybody explain what's the significance of this variable? I thought just checking username and password is enough for authentication.

Thanks.