comments-template.php function comments_popup_link() parameters explanation

Hey, can anyone explain what the parameters of the function comments_popup_link() (that is in the comments-template.php file) mean and why the parameters are passed like _( , ) some kind of list. Is that just php syntax for passing in lists? I want to be able to customize this method to just output the number of Comments, not Comments and Reactions like it does in the twentyten theme. Thanks.