for they know not what they do,or "are doing", meaning, in crucifying him, which was the case of many of them, and of their rulers; they did not know that Jesus was the Messiah, nor the prophecies concerning him, nor the evil they were committing in putting him to death: not that their ignorance excused their sin; nor was it without sin; nor does Christ use it as a plea for pardon, or found his intercession upon it, which is always done upon his own propitiatory sacrifice; but this is mentioned as descriptive of the persons Christ prays for, and points out a branch of his priestly office he exercises, in having compassion on the ignorant, and them that are out of the way; (John Gill's Exposition of the Bible; Luke 23:34)
In this case the verses need no elaborate commentary. Jesus asked for their forgiveness. The sin shall not be charged to them. Not that they will not be charged with every other sin they committed, but this one was "on the house".