Chapter 5, Mishnah 2: There were ten generations from Adam until Noach. This is to teach us the extent of G-d's tolerance, for all these generations angered Him until He brought upon them the waters of the Flood. There were ten generations from Noach to Avrohom. This is to teach us the extent of G-d's tolerance, for all these generations angered Him until Avrohom came and reaped the reward for them all.
The Simple Explanation
Rabbi Ovadiah of Bartenura explains this Mishnah as follows. The Mishnah tells us of two groups, each spanning ten generations. First, it tells us of the ten generations beginning with Adam, the very first generations in history. The tenth generation was that of Noach. The reason for telling us of these generations is to demonstrate how great Hashem's patience is. Even though each of these generations angered G-d, He allowed ten such generations to pass until acting. It was only after many hundreds of years that Hashem brought the waters of the Flood upon them.
This teaches us an important lesson. Throughout history, G-d has allowed many nations to thrive. This is so even though they mistreated His children, the Jews. From this Mishnah, we can understand it is due to His great patience.
The second group consisted of the ten generations that spanned from Noach until Avrohom. Regarding those generations, we say that Avrohom reaped the reward for all of them. Avrohom single-handedly performed all the goodness and kindness demanded of the nine generations preceding him. Hence, he received all the rewards they would have been entitled to had they behaved following Hashem's wishes. This is in keeping with the teaching of the Talmud that "each person is born with two portions; one in Gan Eden and the other in Gehinnom – Purgatory. If one merits, he receives his portion and that of his fellows in Gan Eden. Otherwise, he receives his portion and that of his friend's in Gehinnom."
See Talmud Chagigah 15, b.
