Albert Barnes Commentary Psalms 86:15

"But thou, O Lord, art a God merciful and gracious, Slow to anger, and abundant in lovingkindness and truth." — Psalms 86:15 (ASV)

But thou, O Lord, art a God full of compassion ... - See the notes at Psalms 86:5. The words rendered “long-suffering” mean that there was and would be delay in his anger; that it was not soon excited; that he did not act from passion or sudden resentment; that he endured the conduct of sinners long without rising up to punish them; that he was not quick to take vengeance, but bore with them patiently. On this account the psalmist, though conscious that he was a sinner, hoped and pleaded that God would save him.

Plenteous in ... truth - That is, in faithfulness. When you have made a promise, you will faithfully keep it.