It is a utility that constantly monitors some of the most popular sites on the internet to see when they are down. Often times it is hard to know if a site is actually down or not, or if it is only you that can't reach the site. If we are not able to reach the site, chances are that the website is having problems and it is not a problem on your end.
This can occur for many reasons. Often it is as simple as your internet provider being down. This could mean that your have no internet at all, or that the DNS servers you are using are unavailable. Another reason a site could be down for only you is if your internet route to the site is lost. The internet is built with redundancy in mind, but sometimes a small segment of the internet can be cut off from another section of the internet.Another common problem can be the configuration of your computer. There are many different settings that could effect your ability to reach one, or many sites.
The possibilies really are endless for this. The site could be doing maintenance, or upgrading their system. They could also have a bug in their system that has resulted in the site going down.Another common problem a flood of traffic. This can either be a good thing or a bad thing. The good would be that the site is becoming very popular and they are getting too much traffic. The site will need to invest in more servers and infrastructure to handle their new found popularity. The bad occurs when something like a DoS (Denial of Service), or DDoS (Distributed Denial of Service) attack occurs. This is when someone uses software vulnerabilies or a botnet to flood the site with traffic. Under these circumstances there is nothing you can do to get the site to come back up, all you can do is wait.