Port scanning is when someone scans your IP address looking for any open ports to dump some kind of virus, trojan horse, ect on your system to hijack it.
Make sure you don't have any unneccesary ports open. You should be behind a firewall and only open the ports needed through the firewall. And password protect any of those that are open.
You can search the web for sites that will scan your system and tell you what ports are open. Just search port scanning test.
Definitly you need a firewall if not already installed.!!!