Threshold (noun):  the place or point of entering or beginning

https://www.merriam-webster.com/dictionary/threshold

“The way we live now” Colson Whitehead

“You didn’t know that each time you passed the threshold you were saying goodbye.”

I have encountered this word many times before and when I see this word I just think of holding to a certain extent, in the sense of ” this is beyond the threshold” but then in this sense, you get to see that the word meant entrance in reference to the idea that you would no longer see this place again. Getting to understand the different context of the word helps to understand the word and sentence in a better sense.