Uptime is a measure of the time a computer system has been “up” and running. Uptime is a great measure of how good a Web hosting provider is at keeping their systems up and running. If a hosting provider has a high uptime percentage, then that means that their servers stay up and running and so any site you host with them should stay up and running too. Since Web pages can't keep customers if they are down, uptime is very important.
There are 2 types of uptime in web hosting industry. One is the Network Uptime, and the other one is Server Uptime.
Network uptime is talking about the availability of the network, which including the network within the data center and the connection out to the Internet. Network is usually very stable, so a lot of web hosting service providers would like to give you a guarantee on it.
And server uptime refers to the availability of your web hosting server from outside the data center. This will directly determine how possible your site will be accessed without any problem.
Considering the fact that most uptime guarantees are given on a monthly basis, if your website was down 10% of the time, it means if was down for about 3 days. If you run a decently visited website a 10% downtime costs you (in loses) way more that the monthly cost of web hosting.
Searching for a 99% uptime guarantee hosting provider is not an easy work on web. As there are a lot of companies who says they offer 100% uptime guarantee which is not possible. So, don't go with their promotion offers which are made to market their product. Uptime is a reflection of how long the machine that is running your website stays up and running. But that machine can be up and working and your website down. If you aren't maintaining the Web server software (and other software like PHP and databases) for your site, you should make sure that your hosting agreement includes guarantees for the software running time as well as hardware uptime.