Definition: To bend a single knee to the ground as a measure of respect or submission
This is a religious term, as there are several specific religious practices around it.
However, it’s more known as an idiom or shorthand to mean “defer.” An alternative definition:
To be servilely respectful or deferential; grovel.
I’ve interpreted it from context quite often to mean, “obviously demonstrate appropriate respect.”
Added on August 22, 2022
This situation is recalled in the book Too Big To Fail:
To break the tension, [Treasury Secretary Hank Paulson] went down on one knee before House speaker Nancy Pelosi.
[…] Pelosi tried to repress a smile at the site of the towering Treasury secretary genuflecting before her…