In short, a grievance is some external event (definition #1) about which you feel aggrieved (definition #2). A grievance says to the world, “You did something wrong to me, and I’m rightfully upset about it.” (It says this especially to other people—the Course’s material on grievances focuses almost exclusively on grievances against other people.)
Full Answer
Each grievance stands like a dark shield of hate before the miracle it would conceal. And as you raise it up before your eyes, you will not see the miracle beyond. Yet all the while it waits for you in light, but you behold your grievances instead. Today we go beyond the grievances, to look upon the miracle instead.
For every grievance is a block to sight, and as it lifts you see the Son of God where he has always been. He stands in light, but you were in the dark. Each grievance made the darkness deeper, and you could not see. Today we will attempt to see God’s Son.