USING
p_EndRow,
p_StartRow;

means

:EndRow will bind the value of p_EndRow and :StartRow will bind to p_StartRow


if you have say a third bind variable then you would have to add another variable after p_startRow in using clause

it's dynamic SQL that's why

go to tahiti.oracle.com and read about pl/sql guide about native dynamic sql, it's all explained there