Eclipse build workspace task stuck

Today I have strange problem with Eclipse. When I try to configure build path of one of my project Eclipse start to build workspace but couldn't finish. Build workspace task just hang.

I have to start task manager (TIP : in Windows you can do that by pressing CTRL+SHIFT+ESC keys)and kill Eclipse process.

I ask Google and find this page http://www.eclipsezone.com/eclipse/forums/t61566.html. There was advice there to run Eclipse with -clean parameter. And it work like a charm :).
But what this parameter do? From Eclipse documentation :

if set to "true", any cached data used by the OSGi framework and eclipse runtime will be wiped clean. This will clean the caches used to store bundle dependency resolution and eclipse extension registry data. Using this option will force eclipse to reinitialize these caches.

Lot of more information you can find on article mentioned above.