What does the Oracle SQL execution plan cost mean?

It is used by the cost based optimizer for comparing different ways of executing the same statement. CBO parses your query, creates possibly hundreds of different ways to execute the query, then chooses the version with the lowest cost. And, if you are experimenting with different codings of a query, you can use Cost to some degree to gauge which version of our query is best. Theoretically, a lower cost plan will be faster than a higher cost plan, but not always. Best to test the various versions to ensure which version is most performant.