If the conquering guns present at any world are strictly more than three times the DP of the conquering ships belonging to the world's owner, then that world is said to be interdicted.  No designs, builds, or repairs can take place at an interdicted world, nor can any RUs be shipped offworld (as per blockades).  This interdiction remains in place from turn to turn until the above condition is no longer true.

Note that it is the total number of foreign guns, regardless of owner, which determines the interdiction; thus, it is possible for multiple empires to interdict a world (possibly by accident) that none of them could interdict alone.  Missiles and devices are not included (by either side) for purposes of determining interdiction.
 
An empire's homeworld cannot be interdicted (although it can be blockaded).
 
Allies: Allied ships count as friendly DP, not as foreign guns for purposes of establishing interdictions.  Note that an interdiction affects allied stockpiles, as well as the owning empire's stockpiles (even if the interdicting empires are separately allied with the owning empire's ally).