Grace is the term used by the angels to describe their own unique abilities and knowledge. It is also sometimes referred to as "divine grace," but shortening it to simply "grace" is far more common. These abilities often include ones that are common among the angels in a choir, such as all Powers being able to sense demons within a five-mile radius, and all angels in general being able to change their form. It also refers to the unique abilities and knowledge of individual angels, as each angel was created with slightly different strengths, weaknesses, and range of known ideas and concepts.
Despite theological opinion, this does not apply in any way to mortals, at least not in a way that comes directly from God to mortals. The only way for mortals to know "God's grace" is through the knowledge passed down from the angels about any number of subjects they would not have learned about without angelic intervention. For example, angels taught mortals how to make bread and domesticate animals, thereby passing on their divine grace to mortals. This is also true for mortals who are graced with knowledge about the angels, demons, God, and the universe directly from angels.