Access paths are relational operator (ways, techniques) that retrieves data from the database.
When talking about the term access, you can also hear for the same term the word scan.
In general, index access paths should be used for statements that retrieve a small subset of table rows, while full scans are more efficient when accessing a large portion of the table. Online transaction processing (OLTP) applications, which consist of short-running SQL statements with high Oracle Database - Selectivity, often are characterized by the use of index access paths.
Decision support systems, on the other hand, tend to use partitioned tables and perform full scans of the relevant partitions.
To locate and retrieve any row in any table, several access path can be used :
How the Query Optimizer Chooses an Access Path
Check the following paragraph : Understanding the optimizer