Hosting uptime refers to the percentage of time the host is accessible via the internet. Many hosting providers state that they aim for a 99.9% uptime, but there may be server restarts and planned (or unplanned) maintenance in any web hosting environment.
A popular claim from the popular hosting providers is '99% or 99.9% server uptime' but this often refers only to a server being powered on and doesn't account for network downtime. Real downtime can potentially be larger than the percentage guaranteed by the hosting provider. Many providers tie uptime, and accessibility, into their own Service Level Agreement, or SLA. SLAs may or may not include refunds, or reduced costs if performance goals are not met.