The describe function is called by the Oracle's SQL engine to query the row shape (that is basically number of columns, their names and data types) that the PTF will return.
The arguments of the PTF and the describe function must be the same, but the table data type of the PTF replaced by the dbms_tf.table_t descriptor type and the columns data type by the dbms_tf.column_t descriptor type.
The describe function can mark (input-) columns as pass through to indicate that they will be kept when producing the output.
Columns marked as read columns are used to compute values that are dependent on these read columns.
The columns that the PTF intends to return is returned by the describe function using a dbms_tf.describe_t descriptor.
If the PTF intends to pass on the same columns as the input table, describe can just return null.