Nmap null scan attack

Does any one know what a Nmap null scan is it just attack my pc I was on the BBC site and Norton just said it was blocked. The weird thing is it happened about an hour aho and 3 days ago after 7 aswell.

 

It says An intrusion attempt by 213.190.36.2(that is the same code that attacked me the other 2 times) was blocked.

 

Is it anything to be worried about or will Norton just block it all the time.