Then he said to Auddâlaka Âruni: 'O Gautama, whom do you meditate on as the Self?' He replied: 'The earth only, venerable king.' He said: 'The Self which you meditate on is the Vaisvânara Self, called Pratishthâ (firm rest). Therefore you stand firm with offspring and cattle.

In plain words: The king asks Uddalaka Aruni what he worships as the Self, and he says the earth. The king says earth is one part of the universal Self, called 'firm support.' That is why Uddalaka stands secure with children and cattle.