Server uptime indicates the amount of time that a web server is fully functional and online. If a server crashes or goes offline, all of the website and email services hosted on it will become non-functional until the problem is resolved. Server uptime is a direct indicator of the reliability and consistency of a web hosting provider.
Most web hosting providers will indicate they have more than 99 percent uptime. Typically a server uptime guarantee will exclude scheduled maintenance. There are always going to be some updates that will interrupt the hosting service. These interruptions are normally brief (just a few minutes), quite rare and take place during hours when websites are less busy (early morning NZ time).
If you have been experiencing frequent downtime then you may want to switch web hosting providers. Every time your website becomes unavailable you are potentially losing visitors and money. This is especially true if you are operating an eCommerce online store website.