What is meant when Tom Brokaw, one of the speakers on the video, says "...on the cusp of something so profound, that it is hard to put it into words."

"...on the cusp of something so profound, that it is hard to put it into words" means that something profound was about to happen at that point in time.

When we can even say for example,  when Tom Brokaw was diagnosed with multiple myeloma in 2013, he was "on the cusp of something so profound, that it was hard to put it into words."

Explanation:

It could also mean when someone is about to enter a new experience either in professional life or otherwise.  When somebody is overwhelmed by the current situation or position, it may be very difficult for the person to put into words the newly found position.  In summary, "on the cusp of something" shows the intensity and profundity of the new situation.

the reason that neiterkob's daughter most likely tell the beginnings of the maasai is : to explain the readers the origin of maasai culture

with this explanation, the reader will grasp a better concept about it and not lost in the middle of reading.

                                            (hope this )