Full Idea
Intuitively, the Barcan formula says that nothing comes into existence when moving from a possible world to an alternative world. The converse says that nothing goes out of existence. Together they say the domain of quantification is fixed for all worlds.
Gist of Idea
Barcan:nothing comes into existence; Converse:nothing goes out; Both:domain is unchanging
Source
Koen Vervloesem (Barcan Formulae [2010])
Book Reference
'Key Terms in Logic', ed/tr. Williamson,J/Russo,F [Continuum 2010], p.8
A Reaction
Stated so clearly, they sound absurd. The sensible idea, I suppose, is that you can refer to all the things from any world, but that doesn't mean they are possible. Shades of Meinong. 'Square circles' are not possible.