“strikes down any person.” More literally “strikes down any soul,” where “soul” is used of the person, and indicates the whole person.
“unintentionally.” The translation “unintentionally” means without premeditation. The Hebrew is literally, “without knowledge,” and includes without premeditation, but it would also include doing something that someone died from but you did not even know the person had died. For example, someone walking on a mountain top might kick a stone that rolls down into a wadi below and hits and kills someone, and the person who loosed the rock might not even know about it.
“avenger of blood.” A member of one’s family or clan that would kill anyone who killed someone in the family (see commentary on Num. 35:19). However, there was a procedure. There had to be two witnesses, etc.
The word translated “avenger” is the word for “redeemer.” The “avenger,” the “redeemer,” provides justice.