A creeper never groweth unless there is a large tree to twine round. O king, O son of Ambika, thy son is as a forest. O sire, know that the Pandavas are the lions of that forest. Without its lions the forest is doomed to destruction, and lions also are doomed to destruction without the forest (to shelter them).

In plain words: A creeper cannot grow without a big tree to climb. Vidura tells the king that his son is like a forest and the Pandavas are its lions. Without lions the forest is destroyed, and without the forest the lions die too.