How it works…

Suppose you change the value of a setting during your session, for example, by issuing this command:

SET work_mem = '16MB';

Then, the following will show up in the pg_settings catalog view:

postgres=# SELECT name, setting, reset_val, source
                       FROM pg_settings WHERE source = 'session'; 
   name   | setting | reset_val | source   
----------+---------+-----------+--------- 
 work_mem | 16384   | 1024      | session

This will show until you issue this command:

RESET work_mem;

After issuing it, the setting returns to reset_val and the source returns to default:

  name   | setting | reset_val | source   
---------+---------+-----------+--------- 
work_mem | 1024    | 1024      | default