Uptime is the amount of time that your website is available and online throughout the month. Thus if your website has 100% uptime it means that your site was not down for any time throughout the month. If your site was down for a total of 45 minutes in the month then your uptime was approximately 99.9%.
Is 100% uptime possible?
Unfortunately no hosting company can provide 100% uptime. Even the biggest and best multi-million rand and dollar sites go down from time to time due to various unforeseen circumstances. You need to make sure that your web hosting company has done the best they can to ensure the least downtime possible. Make sure that they have systems in place to continually monitor all of the websites they host so that they know if there is a problem as soon as possible.