If the foreign conquering guns present at any world are strictly more than twice the DP of the conquering ships belonging to the world's owner, then that world is said to be blockaded.  No RUs can be shipped offworld from any blockaded world; this restriction remains in place from turn to turn until the above condition is no longer true.  A world that is interdicted is also blockaded, but the reverse is not necessarily true.
 

Note that it is the total number of foreign guns, regardless of owner, which determines the blockade; thus, it is possible for multiple empires to blockade a world (possibly by accident) that none of them could blockade alone.  Missiles are not included (by either side) for purposes of determining blockades.
 
 
Allies: Allied ships count as friendly DP, not foreign guns, for purposes of determining blockades.  Note that a blockade affects any allied stockpiles, as well as the owning empire's stockpiles (even if the blockading empires are separately allied with the owning empire's ally).