What fundamental truth about human existence did Sai Baba want to convey by constantly giving out Udi from the sacred Dhuni?

The fundamental truth Sai Baba conveyed by distributing Udi, as explained in Chapter 33 of the Shri Sai Satcharita, is the impermanence of the material world and the ultimate reality of Brahman. The Udi symbolizes that the entire visible universe, which is pervaded by Maya (illusion), is transient and will ultimately become ash. This lesson extends to our own bodies, which are described as 'wood made of five elements' that will also become ash. Chapter 33 emphasizes that the Udi is a reminder that 'we came naked and will go naked,' encouraging the development of discrimination and renunciation by detaching from worldly bonds.