Somewhere on the Internet, I read that we can use text files as virtual tables (i.e., we can read/write data from/into the text file by mounting the file). What does "mounting a file" mean?
Starting in Oracle9i, you can reference a text file that is external to the database just as if it were a table in the database. This concept is called an external table. Basically, you create a table that will access the external text file. You can then issue SELECT statements against this external table. These external tables are READ ONLY. You cannot modify their contents.
For more information, please refer to the Oracle documentation on external tables. The docs give you a complete, step-by-step example on how to create an external table.