Albert Barnes Commentary Psalms 103:8

"Jehovah is merciful and gracious, Slow to anger, and abundant in lovingkindness." — Psalms 103:8 (ASV)

The Lord is merciful and gracious - See the notes at (Psalms 78:38). The idea here is evidently derived from (Exodus 34:6–7)—that great and glorious statement of God himself regarding his own character. Our world is a different world under that statement than it would be if that and related declarations had not been made.

There is here a progression of thought, an advance on the previous statements. At first, the psalmist referred to his own individual experience (Psalms 103:3–5); then he referred to the dealings of God toward the Hebrew people (Psalms 103:6–7); and now he rises to the general contemplation of his character as it relates to all mankind. It was a characteristic of God in respect to all, that he was kind, compassionate, and forbearing.

Slow to anger - That is, patient; not soon excited; bearing much, and bearing it long. See (James 5:11); .

And plenteous in mercy - Margin, "great of mercy." The Hebrew word means "much" or "great"; and the idea is, that mercy is not manifested by him in small or stinted measure. It is rich; full; abundant; overflowing; free.