What is the meaning of "Kadeshu Tzom"?
Rashi, Targum Yonasan: Declare a fast. Malbim - the Kohanim should do so.
Radak: Summons the nation for a fast.
What is "Atzarah"?
Rashi: It is a gathering.
Radak citing Targum Yonasan: Call a cessation - people will cease work and gather to scream to Hashem. Perhaps He will hear your screams and remove this plague from you! The Navi tells them to do so when the locusts come, or to repent beforehand, and Hashem will nullify the decree in His mercy.
Malbim: Kohanim should gather the nation to Beis Hashem. They will gather the Zekenim and all residents of the land to Beis Hashem to scream to Hashem. At the time of fasts over afflictions, they gathered the nation and bring the Zaken in front of the Aron, and he expounds in front of them words to humble them (Ta'anis 2:1).
