Implicit block?

I have just noticed this appearing in my NAV2008 activity log

 

 

Details:

Rule "Implicit block rule" blocked (0.0.0.0,bootps(67)).
Inbound UDP packet.
Local address,service is (255.255.255.255,bootps(67)).
Remote address,service is (0.0.0.0,bootpc(68)).
Process name is "N/A".

 

Any idea what this means? I recently unchecked the trust local networks option in

realtime protection

  Internet worm protection

If that plays a role in this