The Summary tab in the Dashboard > System page displays information on TRAFFIC DISTRIBUTION, TOP USERS, OBSERVED THREATS, and TOP DEVICES BY SESSIONS in your network infrastructure, for the period selected in the slider at the top.
TRAFFIC DISTRIBUTION: Shows the graphical representation of the percent distribution of the number of network sessions based on protocol.
TOP USERS: Shows the top users by the number of sessions, amount of data received, amount of data sent, and the number of blocked connections.
OBSERVED THREATS: Shows the different types of threats and the number of threats of each threat type across managed devices.
TOP DEVICES BY SESSIONS: Shows the list of devices that are sorted in descending order of the category you select. Click the Gear icon to select your desired category; the default selection is Sessions.
The Insights section (scroll to the right if it's not visible) gives information about number of infected hosts and the number of critical attacks.