Explain this sed command

From: boobie (boobie.webdeveloper_at_mailnull.com)
Date: 11/11/05


Date: Thu, 10 Nov 2005 23:47:05 -0500

sed -i "s/<port>/$LISTENER_PORT/g" $ORACLE_HOME/network/admin/tnsnames.ora

Please put the above in plain (technical ;) ) English

thanxks