$$ in postgres

$$ in postgres is a string literal. Function bodies in postgres (and sql) as plain text strings. Check out this example from a previous post on stored procedures:

create or replace procedure report_status_update(
  new_status varchar(20),
  report_id int
language plpgsql   
as $$
   update reports
   set status = new_status
   where id = report_id

You will notice we capture the actual function body (starts with the begin keyword) in $$ and after we have finished defining the function body (ends with end keyword) then we close the string literal with another $$.