Uptime is the amount of time (in days, hours and minutes) the server, network, or website has been running (up). This is usually listed as a percentage, like 99.9% uptime for a given period of time.
The uptime for a server, network, or a website can be viewed under Availability, above the Events Timeline in the web client.
System uptime is the amount of time indicating the duration (in days, hours and minutes) for which the system has been operational. This can be viewed by navigating to Server > click on any server > Summary > System Statistics > Uptime