וַאֲבָֽרֲכָה֙ מְבָ֣רְכֶ֔יךָ וּמְקַלֶּלְךָ֖ אָאֹ֑ר וְנִבְרְכ֣וּ בְךָ֔ כֹּ֖ל מִשְׁפְּחֹ֥ת הָאֲדָמָֽה׃
너를 축복하는 자에게는 내가 복을 내리고 너를 저주하는 자에게는 내가 저주하리니 땅의 모든 족속이 너를 인하여 복을 얻을 것이니라 하신지라
Rashi
ונברכו בך AND IN THEE SHALL BE BLESSED — There are many Agadoth concerning this but the plain sense of the text is as follows: A man says to his son, “Mayest thou become as Abraham”. This, too, is the meaning wherever the phrase ונברכו בך “And in thee shall be blessed” occurs in Scripture, and the following example proves this: (Genesis 48:20) בך יברך “By thee shall Israel bless their children saying, “May God make thee as Ephraim and Manasseh”.
Chizkuni
ואברכה מברכיך, “I will bless those who bless you.” You must never think that there are no people on earth that are psychologically close to you and are potential saviours of you, for I love those who love you and I hate the people who hate you. [in other words I know that there are people who love you. Ed.]
Da'at Zekenim
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.”