While I could turn on Rewrite Logging and mess with this rule to figure out what it does (if anything), I bet someone already knows. Here's the rule by itself:
RewriteRule ^ - [L]
Here it is in context:
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
I guess it means "Match Everything". I'd have thought that "Match Everything" would be a dot to make it more obvious that there is no missing code, and that regexp would error out if the caret was the only thing in it. So I see the outside chance that it means something else.