The numbers that a potential client should be looking for are the maintenance schedule as well as the mean time to recovery or some equivalent. Every server has to be taken off line and all data should be backed up; the maintenance schedule will tell you how long your site is offline in order to do this and usually when. Note that a site can be up while maintenance is happening; the site just cannot be modified while it is ongoing, and it depends on what kind of server configuration it has. Nonetheless, you should expect the site to be down for at least a few hours a month in order to maintain the servers; any more than that and you should debate someone else.
Most of the numbers are used solely to attempt to look good and, while perfectly legitimate numbers, are only useful to experts. What you want to know is how fast the servers will be back up in case something happens. The ideal is just a few minutes, but as long as thirty minutes is acceptable. Anything longer and you should really debate someone else, as it means they have little in the way of back-ups. By looking at the SLA and studying it, you can tell what kind of company you are dealing with so pay attention to it.
You can keep track of some of this yourself, and you should, however informally. The biggest thing that you should note a difference in your numbers; a drop in numbers with no other explanation means that the site is down more often than it should be, and that you should look into the performance of your website's numbers; the logs and visitor counts should clue you in to when and how often the site is down. If it is down too often you may wish to switch your webhosting company. It is your site; if you are having problems you may wish to do something about it and as quickly as possible.