The given query is using the CROSS JOIN clause, which produces a Cartesian product of the tables involved.
The DUAL table in Oracle is a special one-row, one-column table present by default in all Oracle database installations. When you cross join the DUAL table with itself multiple times without any where clause to limit the rows, the result is a multiplication of the row count for each cross-joined instance.
Since DUAL has a single row, cross-joining it with itself any number of times will still result in a single row being returned. The SELECT 2 part of the query simply dictates that the number 2 will be the value in the column of this single row.
So, the result of this query will be one row with a single column containing the value 2.
References:
* Oracle Database SQL Language Reference 12c, especially sections on join operations and the DUAL table.