Server Uptime refers to the time that a web server is fully functional since it came online. Web Hosting providers advertise high Uptime, when in reality you would want 100% Uptime Guarantee. With that guarantee, you should know that Hosting firms have redundancy proof backup servers, generators, power-supplies and related tech to keep their servers online all of the time.
If a server crashes or goes offline, all websites hosted on it are rendered useless until operations are restored. So, Server Uptime is a measure for checking the reliability of a hosting. In case you are experiencing frequent downtimes (server crashes and stops) you should check with your provider if they live up to their advertised Server Uptime standard or switch providers they do not.