Can anyone please tell me what does below Stored Procedure does?

I am a newbie oracle PL/SQL stored procedure.Can anyone please tell me why put "g_detail_select||" in a query place and What is  ||?I do understand after  for I should see a query statement but why "g_detail_select||"

procedure get_activity_details (
    in_twp_user_id                    in twp_user.id%type
    ,in_activity_id                in activity.id%type
    ,out_retcode                       out number
     ,tmp_rs                            in out rs_cursor) is

begin
    out_retcode := 0;
    open tmp_rs for g_detail_select||' where a.id='||in_activity_id;
end;

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