The Risk-Level describes the current phishing-, malware- and spam-situation that we find on the Internet. It gets updated every 15 minutes.
The green level 1 means that less than half of the malware or phishing sites is currently active (in the last 24 hours) compared to the average of the last four weeks. Yellow means a steady level, and red (5) means more than double of active malware samples, phishing sites or spam emails – a so called outbreak takes place and users should be more careful when using the Internet.
Sorin Mustaca wrote a more detailed Blog article which describes the Risk Levels in-depth.