It’s the name/reference for your widget, what you’d previously have used for the first parameter in register_sidebar_widget, with wp_register_sidebar_widget the name(second) parameter sets the display name..
So put simply, it’s just a reference name/slug for the Widget, with the second parameter being the display name.
Compare the two pages and see for yourself.. 😉
http://codex.wordpress.org/Function_Reference/wp_register_sidebar_widget
http://codex.wordpress.org/Function_Reference/register_sidebar_widget
Thank you.
I will think of a descriptive and therefor unique slug.