What is "ha'Arar"?
Rashi #1: It is one who screams - "Za'akas Shever Ye'oreru" (Yeshayah 15:5).
Rashi #2: It is destruction - "Aru Aru" (137:7).
Radak: It is a tree that grows in the Midbar. Yisrael in Galus are like it.
Malbim: It is an individual.
Why does it say "v'Lo Vazah Es Tefilasam"?
Radak: The nations think that He despised their Tefilah.
Malbim: Even though my days are like a shadow, and I will not see the Ge'ulah, I know that this Tefilah that I pray for the Ge'ulah, even though it is Tefilas ha'Arar, Hashem hears it ("Shim'ah Tefilasi") - all Tefilos at the time of the Churban, whether of an individual or a Rabim, Hashem does not despise them. He hears all of them, gathers them and guards them for the designated day.