1)
What is "Mashal"?
1.
Rashi: It is Divrei Torah, which are called "Mashal ha'Kadmoni" (Shmuel I, 24:13).
2.
Radak: I will speak in the way of a parable and riddle.
3.
Malbim: It is a parable of redeeming a brother, and redeeming oneself from death. It is a parable for redeeming the Nefesh from its own death.
2)
What is "Chidasi"?
1.
Rashi: It is what the next verse says - why must I fear on days of Ra?
3)
Why does it say "Eftach b'Chinor Chidasi"?
1.
Radak: A player will play the harp together with these words, which are a parable.
2.
Malbim: Amidst playing on the harp, he will immediately solve the riddle and understand the Nimshal.