This name has been repeatedly mentioned in many verses. Ar-Rabb is the one who nurtures and sustains all of His servants through regulating and governing their affairs and granting them all types of favours and blessings. More specifically, He is the one who nurtures and sustains his sincere friends by correcting and purifying their hearts, souls and manners. This is why their supplications are frequently made with this noble Name because they seek this specific nurturing.
“And your Rabb is the Ever-Forgiving, Owner of Mercy. Were He to call them to account for what they had earned, surely He would have hastened on their punishment. Instead they have their appointed time beyond which they will find no escape.”
[al-Kahf (18): 58]
From “Explanation to the Beautiful and Perfect Names of Allah” — Shaykh as-Sa’di (d.1376H)