Its probably just some script kiddie somewhere who is scanning IP ranges to see if you have any open ports.
They usually scan for specific open ports, - most commonly those which are used by certain trojans, (or server ports). If you say for eg had a trojan on your system, then it would broadcast itself and the wannabe hacker would know that you have a weakness on your network which he could then attempt to exploit.
The fact that your router has picked this up is good and is just a notification to you. Unfortunately this type of activity is rife on the internet and its something most of us get.
When I was with BT many years ago I found that I used to get on average at least a dozen port scans per day... sometimes a hell of a lot more. For some reason they see the more common ISP ranges as being easy targets where more novice users are on-line. Since being with PN, I still get them on occasions (although tbh I've now turned logging off).
You can complain to the ISP of source of the attempt - and whilst UK/US ISPs would look at this and likely issue their users with a warning/cut their connection.... the chinese and russian ISPs tend to be very lax and do nothing, which is why its a complete wast of time reporting it.. (and why I actually turned off my logging).
This sort of activity is a prime example of the background noise on the internet that increases bandwidth utilisation (eg your query the other day about why your bandwidth usage was xMB when your machines hadnt even been switched on).
Theres a bit more info here if you want to read
http://www.tamingthebeast.net/articles/scriptkiddies.htm
