a association is a table with 2 columns : object1ID , object2ID
based on the association type (one to one, many to many etc) this table is filled
you can find these tables in postgres with : db -> schemas -> public -> tables -> <moduleName>$<associationName>
So when you use a Xpath with this association to another object Mendix translates that xpath with this intervening table into the query.
As per database concepts association is similar to the the foreign key relationship between two tables.