The L-Flag stopps processing immediately, rules below are not applied anymore.
RewriteRule ^/old/(.*) /new/$1 [L] RewriteRule ^/new/$ /new/index.php [L]
A request of /old/ would be rewritten to /new/ by the first rule but with the L-Flag the second would would not be checked anymore. This saves processing time, if you know, that rules below shouldn't apply anymore against the substituted result.
Note: If a substitution occured in per-dir context, an internel redirect would take place. This means that all rules might be processed again, if the redirect processing reaches the per-directory configuration again.