Below Image: Many Web Hosting Provider mentions 99.9% Up-time. It is amount of time your website is online without going offline.
Downtime is amount of time website is offline.
If web hosting provider providing highest up-time , it means that there server keeps up and running so any website on that server can stay up and running.
For example, if some host has an up-time average of 99.86%, this means that your site will be down for a total about 1 hour each month.