Generally speaking (but without knowing the context of how you heard the term...), web site availability refers to the amount of 'uptime' of a website.
In many cases, this is related to Service Level Agreements for hosted sites - there might be, for example, a contractual guarantee of 95% uptime for the website, which would be defined as the web site availability.
In some cases, however, this refers to the availability of the web site by certain browsers or configurations of platform and browser. It all depends on how it was being used, really.
------------------ To affect the quality of the day; that is the highest of arts