Table access by index rowid : what does it means ?

Searching the index is a fast and an efficient operation for Oracle and when Oracle finds the desired value it is looking for, it can also find out the rowid of the record in the corresponding table. Oracle can then use this rowid to fetch further information if requested in query.

All indexes store the rowid of the row indexed as part of the index. That is why an index can speed up access to the table, since typically the index is much smaller than the table, and is stored sorted by the value of the indexed colum

The "TABLE ACCESS BY ROWID" step does not mean that an index is used. It means that data is read from the table using the ROWID which was (in generally) previously read from an index.