The columns in the source datastore are linked to the columns in the target datastore through mapping.
The easiest way for me to understand an ODI interface is to image that it is an INSERT – SELECT SQL statement. It selects data from the source, which can be one or more tables, and insert to one target table.
Internally an interface may not be that simple. It can perform the behavior like the SQL MERGE statement, to insert a row when the row does not already exist and can update the existing rows when they already exist. It can across different database and even different database platforms.
An interface can be added into a package as a package step.