This is an example of a synecdoche—a part representing the whole. The righthand side was considered the more important and favoured side (e.g., Genesis 48:17–20; Matthew 25:34, Matthew 25:41). Smearing blood on these body parts communicates that the priest belongs, in his entirety, to the service of the Lord. One commentator emphasizes the meaning of this by saying, “The priest must have consecrated ears ever to listen to God's holy voice; consecrated hands at all times to do holy deeds; and consecrated feet to walk evermore in holy ways” (Dillamann in Wenham p.143).
23 And he killed it, and Moses took some of its blood and put it on the lobe of Aaron’s right ear and on the thumb of his right hand and on the big toe of his right foot.