ऋषिभिर्बहुधा गीतं छन्दोभिर्विविधैः पृथक् ।ब्रह्मसूत्रपदैश्चैव हेतुमद्भिर्विनिश्चितैः॥
ṛṣibhir bahudhā gītaṃ chandobhir vividhaiḥ pṛthakbrahma-sūtra-padaiś caiva hetumadbhir viniścitaiḥ
Word by word
The verse says
The sages have sung this in many ways, in various hymns, and in the aphorisms about Brahman — carefully reasoned and settled.
In plain words
He names his sources.
The hymns. The aphorisms. People who worked this out before and wrote it down.
Hetumadbhir viniścitaiḥ — with reasoning, and settled. He is pointing at argument, not revelation.
Where you've felt this · in creative work
A luthier building a guitar is not inventing the shape. The curve of the body was arrived at by hundreds of people over three centuries, most of whose names are gone. He can explain why every part is the size it is, and he did not work out one of those reasons himself.
Going deeper
Pointing at a tradition of reasoning rather than claiming direct revelation is a notable move for a speaker who, two chapters ago, showed Arjuna the universal form.
He could simply assert this. He has the standing. Instead he says that the sages have sung it in many ways and that it has been worked out with reasoning and settled. The authority being invoked is cumulative and argumentative.
The mention of the aphorisms about Brahman is also a scholarly gesture — these are the compressed teaching-formulas the philosophical schools were built on.
What this gives a reader is permission to treat the chapter as something to think about rather than only to accept. The material ahead was arrived at by argument, over a long time, by people who disagreed with each other, and the text says so before it begins.