A filter sub-clause is a sub-clause that references a single table and is not used to determine the access path (it references a single table and does not match the criteria needed for a constraint or access sub-clause).
When the Query Processor has identified the filter sub-clauses, it adds them to an internal list. In a later phase of the optimization algorithm, the Query Processor will assign each filter sub-clause to the filter condition of the table that it references. The filter condition of the table is then used during execution to determine which rows of the table will be included in the result set.