$touser[username] is just the way they call variables from strings. $ denotes a string containing an arbitrary number of variables. That's used pretty much exclusively in the email parser.
for example: $bbuserinfo[username] parses back the information the board knows about the login credential *name* attributed to the account. The $touser[] thing is used when the board needs to figure out who it's talking to.
However under the theme layout system here one *can't* call functions like these.(Disabled for security.)
HTML Code:
{userid}, {username}, {musername}, {sessionurl}, {sessionurl_q} $bbuserinfo[username]
PHP Code:
{userid}, {username}, {musername}, {sessionurl}, {sessionurl_q} $bbuserinfo[username]
Code:
{userid}, {username}, {musername}, {sessionurl}, {sessionurl_q} $bbuserinfo[username]
For more in depth data on escape characters please go here: http://php.net/manual/en/regexp.reference.escape.php
In a nutshell it's a character sequence that tells PHP that the next character or sequences of characters should be interpreted as ascii text instead of code to be worked on.