What does "Clear Of Clouds" mean in reference to Class B Airspace in the United States?
That specific phrasing means that you can't touch a cloud. See the VFR minimums in 14 CFR 91.155, where it says:


  no person may operate an aircraft under VFR when the flight visibility
  is less, or at a distance from clouds that is less, than that
  prescribed for the corresponding altitude and class of airspace in the
  following table [...]


The table lists the "distance from clouds" in class B as "clear of clouds".

You can also find it in the AIM section 3-2-3 on class B airspace, and in other sections about SVFR, visual approaches etc. where a requirement for those procedures is to remain clear of clouds.

The actual cloud cover in the class B airspace is reported by METARs and ATIS.