what is the use of do_action( 'twentytwelve_credits' ) ?

hey friends…..i found do_action( ‘twentytwelve_credits’ ) in Twenty Twelve wordpress theme.but i want to know that what “do_action( ‘twentytwelve_credits’ )” does even i have seen this in other themes also like do_action( ‘xyz_after_body_content’ ) and do_action( ‘xyz_before_body_content’ )….please help to ..thank u in advance.