Uptime is when your website is "live" - i.e. people can see it. Downtime is when your website is "dead" - i.e. people can't see it. Keeping your website up, and knowing when your website is live is important if you want to avoid losing revenues or damaging your brand and reputation. Many people will use a free website monitoring service to alert them to when their site is down so that they can remedy the problem asap.