Downtime is when your site does not work. If the site does not work - you'll not get any benefits from its existence. Downtime is opposite to uptime. There could be various reasons why a site does not work. Most usual cases: problems with server, server overload, application error, network problems and so on. The first step of fixing en error is detecting. There are special instruments, called website monitoring services, which may check sites for errors over the time. Such tools help you to detect error and provide primary analysis of its reasons. Also, usually some information could be found in server logs. If you have access to them, you should review. If not - ask your hosting company to provide them. Detecting of errors may help you to evaluate the reliability of your site and hosting.