It says in Bereishis 12:2-3, “And I will make you into a great nation and I will bless you, and I will make your name great, and you will be a blessing. And I will bless those that bless you, and the one that curses you I will curse; and all the families of the earth will be blessed through you.”
It states in the Tosefta (Sofrim 21:9) that the “person great among giants” mentioned in Sefer Yehoshua 14:15 is referring to Avraham Avinu, whose eating and drinking was comparable to that of seventy-four people. Who are these seventy-four people? It says in Shemos 24:1 that when Klal Yisroel were encamped at Har Sinai, at the giving of the Torah, Moshe, Aharon, Nadav, Avihu and the Seventy elders were at the mountain and they saw Hashem and ate and drank, meaning they basked in the glow of the Shechinah. The Vilna Gaon says that these seventy-four people who “ate and drank” were the one’s equivalent to Avraham’s eating and drinking.
Reb Avraham Mordechai Alter the Rebbe of Gur brings a different explanation, that the seventy-four people correspond to those individuals mentioned in the Torah from Noach to Avraham. It says in the Mishnah Avos 5:2; that Avraham’s reward was equal to the ten generations from Noach until Avraham.
Let me now explain the words “and you will be a blessing” and “I will bless those who bless you.” In transmitting Brachos to people, it has the effect to rebound back to the original person who gave the Bracha. The Zera Shimshon gives insight to this rebound factor. It says in the Gemara Bava Kamma 92A that the one who davens for his friend, while needing the same thing will be answered first. The main idea is that if Hashem answered his Tefillos on behalf of his friend it will rebound back to the original supplicant. However, if Hashem did not answer the person’s Tefillos on behalf of his friend, then he too will not receive whatever it is that he lacks, although he will be rewarded for his efforts on behalf of the other person. What we see is that this is a two-way street, that receiving salvation is dependent on the second individual’s salvation.
The lesson we must take from here is that when one is davening, he should have in mind for others with total Ahavah for the individuals that he is davening on their behalf, similar to the Birchas Kohanim where the bracha is made with Ahavah. By always having in mind our fellow brethren, we can bring about true Achdus among Klal Yisroel.