(first post here brrrrrr)
To my knowlegde, in a tag cloud, the font and boldness of words are chosen according to their rank in the cloud. Some tags clouds even use colors. It works with a simple hierachy like :
16 not bold
15 bold
15 not bold
14 bold
etc...
so this means bold here just means ... nothing special. It's just combined size+boldness you want to pay attention to. For instance, the first keyword is always on the biggest font (though not bold)
Hope this helps (and that I'm right)
Marc