refers to thos occasions when, for one reason or another, a site is offline.
Usually downtime is the result of problems on a server. Most big web hosts have servers located in special facilities (which may or may not be managed by them) where there are arrangements for rebooting servers that go offline. Often the servers themselves have self-diagnostic software installed, and this monitors the performance of hardware components. A failing hard drive usually exhibits warning symptoms and these are detected by the software which can be configured to automatically dial an engineer's mobile phone, so that someone can be onsite, no matter what hour of the day or night. Some facilities are staffed right around the clock.
Server failure is rare, and most professional web hosts can point to an 'up time' performance record in the high 90 percent range - 98% is common. On some packages - mainly the more expensive business ones - you'll be offered a service level agreement, guaranteeing you an up-time percentage.
Many people don't know if their site has been offline for a short while - it happens at night, perhaps, and they're non the wiser. Web hosting companies have to carry out server maintenance from time to time, and they'll write to their customers warning them that the server will go offline for say half an hour on a certain date. Bigger hosts will arrange for server 'mirroring' to prevent any downtime for maintenance - all a server's sites are transferred to another machine for the duration of the work; the sites don't go offline at all in these circumstances.
Does that help?