explain plan for the use of execution plan

Add before the statement to be executed explain plan for.
E.g:

explain plan for
select product_namae,price
from product where id ='111';

Then view the execution plan:

Select * From Table(dbms_xplan.display);

Check the wrong way of writing the execution plan

select dbms_xplan.display from dual;  错

This is wrong.
Return a list of display values "".""(), and I don’t understand what it is.