Genèse 12:3

"And be blessed through you" - there are many Legends and Traditions on this idea and this is their essence: A man will say to his son 'Be like Avraham'. And this is the meaning of all expressions of 'be blessed through you' in Scripture. And the proof is (Genesis 48) 'Through you shall Israel be blessed, May God make you like Ephraim and Menasheh'.

And I will bless the priests who will spread forth their hands in prayer, and bless thy sons; and Bileam, who will curse them, I will curse, and they shall slay him with the mouth of the sword; and in thee shall be blessed all the generations of the earth.

ונברכו, a composite, from the word מבריך, מרכיב, refining through adding superior ingredients. The Torah chose a passive mode to express this blessing.

ואברכה מברכך, your allies and those who seek your welfare.” A reference to Oner, Eshkol, and Mamre, Avram’s allies, as well as others whom the Torah has not named. The blessing‘s effect is that people displaying sympathy and love for Jews will be recompensed by G’d. A prominent example of this is that the house of Potiphar was blessed as a result of Potiphar’s positive attitude to Joseph, his Hebrew slave. (Genesis 39,5) Even Yaakov’s arch enemy Lavan, admitted that his presence with him had resulted in G’d making him rich. (Genesis 30,27)

ומקללך אאור, it is not customary to use the plural mode when speaking of curses. Seeing that there would be very few people, if any, who would curse Avram, this is hinted at in the use of the singular mode in the word ומקללך, “someone who curses you.”

ונברכו בך כל משפחות, all the families of the earth within whose radius Avram would make his residence, such as the land of Canaan, Egypt, and the land of the Philistines, would experience a special blessing due to the proximity of Avram.

ואברכה מברכך, “I shall bless those who bless you.” According to Midrash Tanchuma, section 4 on our portion, G–d had told Avraham that from him would come forth a tribe who would bless His (G–d’s) children. He referred to the priests who would come forth from Aaron and who would bless the Jewish people, as is written in Numbers 6,27: ושמו את שמי על בני ישראל, ואני אברכם, “so shall they put My name on the Children of Israel;” when Avraham asked: “who will bless them (the priests)?” G–d answered: “I shall bless them.” I believe that this is the correct interpretation of what the Talmud tractate Chulin folio 49 meant when the question was asked: “whence do we know that the priests themselves will be blessed, and the Talmud pointed to our verse as the answer to this question, i.e. “I shall bless those who bless them.”

ונברכו בך כל משפחות האדמה, “and all the families of the earth shall count themselves blessed if they can establish family ties with you.” The root of the word ונברכו is the same as that of מבריך, “to graft.” They will consider it an accomplishment to have Abrahamitic blood in their veins.