it means, a SYN flood was detected, and base on the signature on KIS, it blocked the SYN flood to your machine.

That's not highly unusual as infected machines will usually scan ranges of IP addresss to propogate an attack.

Hence, a machine...specifically Windows Machine without any AV, or FW turned on connected to the internet could be infected, owned without even the user having to download any file, click on any link etc.