The credentials is an array that has 'user_login', 'user_password', and 'remember' indices. If the credentials is not given, then the log in form will be assumed and used if set.
The various authentication cookies will be set by this function and will be set for a longer period depending on if the 'remember' credential is set to true.
Note: wp_signon() doesn't handle setting the current user. This means that if the function is called before the {@see 'init'} hook is fired, is_user_logged_in() will evaluate as false until that point. If is_user_logged_in() is needed in conjunction with wp_signon(), wp_set_current_user() should be called explicitly.