It's the formal title for the gods of the Seven. The Dendro Archon is the God of Wisdom who reigns in Sumeru, and indeed, the energy of Dendro is found primarily in plants. Sumeru's eastern section, where the majority of the recorded population live, is a dense rainforest, and Sumeru City itself is built upwards around a giant tree.

[He can only offer academic knowledge of Sumeru, unfortunately. He's never actually been beyond Fontaine's borders.

At her question, he looks a bit sheepish.]

Although I am a dragon in nature, I have always been possessed of a humanlike form - which is, admittedly, most unusual. There are few true dragons left in the world, but the most famous of recent history, the Anemo dragon known on Mondstadt as 'Stormterror,' is far more in line with traditional depictions.