Thy hundred sons and Karna and the sons of Pandu can together rule the vast earth with the belt of the seas. O king, the Dhartarashtras constitute a forest of which the Pandavas are, I think, tigers. O, do not cut down that forest with its tigers! O, let not the tigers be driven from that forest! There can be no forest without tigers, and no tigers without a forest. The forest shelters the tigers and tigers guard the forest!

In plain words: Vidura says Dhritarashtra's hundred sons, Karna and the Pandavas could rule the whole earth together. He compares the king's sons to a forest and the Pandavas to the tigers living in it. The forest shelters the tigers, and the tigers guard the forest, so destroying either one ruins both.