John Gill Commentary Joshua 20:3

"that the manslayer that killeth any person unwittingly [and] unawares may flee thither: and they shall be unto you for a refuge from the avenger of blood." — Joshua 20:3 (ASV)

That the slayer that killeth [any] person unawares, [and]
unwittingly, may flee thither
Who through mere accident, and without design, killed a person, friend or foe, one of his own kindred, or a stranger, without any malice against him, or intention to take away his life:

and they shall be your refuge from the avenger of blood ;
from any of the relations of the deceased, who might be stirred up to avenge the blood of his kinsman on the slayer; see (Numbers 35:12) .