Asking for the action of this 'for-loop'

I could understand that it is iterating the buckets_ , but what is the action in this for loop? In my debugging, if (it->second) is satisfied if (this->next()->putq(it->second) == -1) is not satisfied. So does this for-loop could only judge if the GADGET fail or not?

 for (map_type_::iterator it = buckets_.begin(); it != buckets_.end(); it++) {
      if (it->second) {
      if (this->next()->putq(it->second) == -1) {
        it->second->release();
        GDEBUG("Failed to pass bucket down the chain\n");
        return GADGET_FAIL;
      }
      }
}