What will be a sign and witness?
Rashi: The Mizbe'ach; it is a sign and a witness between them and Hashem.
Radak: As long as they retain Emunah in Kel, they will call to Him and He will save them from all their oppressors.
Malbim: The Matzevah is a sign for Hashem. Via it, they recall that Hashem fights their battle and saves them from their afflicters. The Mizbe'ach is a witness for them, to warn them in the future to serve Hashem, and offer Zevach and Minchah to Him.
What is "[Moshi'a] va'Rav"?
Rashi: An officer.
Radak: Rav is Gadol (someone esteemed), like "Al Kol Rav Beiso" (Esther 1:8). I.e. the savior will be great - Hashem's angel. It will strike their haters, and they will be saved.
Radak citing Targum Yonasan: A judge. I.e. he will judge their haters.
