The RewriteCond directive just describes an additional condition for a RewriteRule directive. So RewriteCond must always be associated with a RewriteRule. Source: http://stackoverflow.com/a/2102189

apache_RewriteCond.conf

; Now this rule is applied if the pattern of the RewriteRule matches the current request 
; URL (per-directory path stripped before) and if the condition is fulfilled.
;
; In this case the condition is only true if when mapping the request URL 
; to the filesystem it matches either an existing file with the file size 
; greater than 0 (-s), or a symbolic link (-l) or a directory (d). So your rule will 
; be applied for any URL (^.*$ matches anything) that can be mapped to something 
; existing in your filesystem. The substitution - just means to not change anything. 
; And the NC (no case, case insensitive, useless in this context) and L (last rule if applied) 
; are flags that modify either the pattern, replacement or the execution of the rule.

RewriteCond %{REQUEST_FILENAME} -s [OR]
RewriteCond %{REQUEST_FILENAME} -l [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^.*$ - [NC,L]