Those are the variables indicated by the syntax $${varname}. In a way, those are not actual variables, but they act as read-only ones. You assign them in one of the FreeSWITCH configuration files (/usr/local/freeswitch/conf/vars.xml), and then they are literally substituted at startup when encountered. This is exactly the same as the #define construct in C preprocessor, or a "Find and Replace" command in a text editor.

<X-PRE-PROCESS cmd="set" data="hold_music=local_stream://moh"/>

In this example, by editing the vars.xml configuration file you alter the value that will be substituted in all configuration files (dialplan included) when "$${hold_music} " is encountered. If you then write in dialplan:

<action application="set" data="mystring=$${hold_music}"/>

it would be exactly the same as if you wrote:

<action application="set" data="mystring=local_stream://moh"/>

Because this is actually a text search and replace procedure that is done once at FreeSWITCH startup, if you change a preprocessor variable in vars.xml you MUST restart FreeSWITCH in order to activate the changes (in this case reloading the configuration without restarting FreeSWITCH is not enough).