Bukhari

Hadith Commentary:
The meaning is that, after Adam (alayhis salam), all the ancestral lineages of the Prophet (sallallahu alayhi wa sallam) were the best families among the descendants of Adam (alayhis salam).
Among your forefathers is Ibrahim (alayhis salam).
Then there is Isma'il (alayhis salam), who is the forefather of the Arabs. After him, among all the lineages of the Arabs, your family was the most noble and exalted.
Your lineage is from the branch of Banu Kinanah among the descendants of Isma'il (alayhis salam), then from Quraysh, then from Banu Hashim.
The duration of a "qarn" (generation) has been stated to be from forty years up to one hundred and twenty years; this is considered one "qarn."
And Allah knows best.

Hadith Commentary:
➊
"Qarn" refers to a group (generation) of people.
The meaning intended by the Messenger of Allah (sallallahu alayhi wa sallam) is that I am from the best group, meaning my father was from the best group of his time, my grandfather from the best group of his time, and so on, until this chain reaches Prophet Adam (alayhis salam).
In other words, it can be said that all the ancestral links in the noble lineage of the Messenger of Allah (sallallahu alayhi wa sallam) were from the best families among the descendants of Prophet Adam (alayhis salam).
Among your forefathers are Prophet Ibrahim (alayhis salam), then Prophet Isma'il (alayhis salam), who is the forefather of the Arabs, and after him, in all the Arab lineages, your family was the most noble and of the highest status. Thus, your lineage passed through the branch of Banu Kinanah from Prophet Isma'il (alayhis salam), then to Quraysh, and then to Banu Hashim.
All these families were known for their good reputation.
➋
Another meaning of this hadith is also stated: that your prophethood was sent in the best of generations (khayr al-quroon), which is superior both in its beginning and its end.