The first Avodah that was done each day in the Beis Hamikdash was Terumas Hadeshen.
One of the Kohanim, selected by a lottery, would go up onto the Mizbeach and take a panful of ashes. He would bring the ashes down the ramp and place them on the side of the Mizbeach, where it would become absorbed on the floor of the courtyard.
In addition to the Terumas Hadeshen, the Kohanim would remove the excess ashes from on top of the Mizbeach. These ashes would be taken to a designated place outside of the camp.
The ashes are the residual that remains after the Korbanos have been burned. The choice parts of the Korban would burn and ascend on High. The ashes that remain are the Pesoles, the ‘waste’ that remains below.
There are two types of Pesoles. One type cannot be elevated and we must distance it completely and remove it from our camp. The other type of Pesoles has some good quality in it that can be refined, elevated and incorporated into the side of Holiness.
In Avodah, these two types of Pesoles are Merirus and Atzvus; bitterness and depression.
One has to serve Hashem with joy. Therefore, both Merirus and Atzvus, being negative emotions, are considered as Pesoles; undesirable states of mind.
But there is a difference between them. Merirus is feeling bad about one’s self and spiritual standing in a way that leads to resolve to change and active steps to grow. There is a positive element within Merirus that ultimately leads a person back to Simcha.
Atzvus – depression, is an overwhelming feeling of lowness that leads to despondency and giving up.
Merirus, like the ashes placed by the side of the Mizbeach that remain within the Azarah, can be elevated and used as a tool in our service of Hashem. Atzvus, like the ashes that were taken out of the camp, must be removed entirely from our experience.
~ Based on Maamarim Kuntreisim volume 1 page 545