Oracle's EXPLAIN PLAN can be used to display the query plan - steps that the database will use to execute the statement. Even though your statement is not a complicated one, you can see that the database will use an index (PK_x_table) when accessing the row you want to delete.
EXPLAIN PLAN is usually used when 'tuning' the SQL statements - making changes that would result into the statement being executed more efficiently. This is actually a vast and somewhat complicated area of expertise, but if you want to get better understanding of it, you might start with this documentation, for example.
As Martin noted, you can see that your query uses and index - which is good. If it were using a table scan, the explain plan would be telling you that you should look into making your query more efficient.