What does it mean "he sprinkled the blood on the people"?
Rashi (citing Targum Onkelos) and Targum Yonasan: He sprinkled the blood on the Mizbe'ach to atone for the people.
Hadar Zekenim: He sprinkled it on the people themselves.
What is the significance of the "Dam ha'Beris"?
Nedarim, 31b (cited in Ein Ya'akov): How great is the Mitzvah of Milah, inasmuch as it is equal to all the Mitzvos in the Torah, as the Torah writes "Hinei Dam ha'Beris asherr Karas Hashem imachem al Kol ha'Devarim ha'Eileh!" 3
Hadar Zekenim: This is like Rashi explained "Ed ha'Gal ha'Zeh" (Bereishis 31:52) - the witness of the Bris punishes one who transgresses it. (Also there he said so in the name of Rashi; it is not in our Rashi - PF.)
See Torah Temimah, note 14.
QUESTIONS ON RASHI
Rashi writes that he sprinkled on the Mizbe'ach to atone for the people. Above (verse 6), he said that the people immersed [because Haza'ah was on them]!
Based on Tosfos Yeshanim (Yevamos 46a): There, Rashi explained like R. Yehoshua, and here like R. Eliezer.