Uman' is a town, 212 km S to Kiev (Ukrainian capital).
Gub is "gubernia" - ~province. (from govern)
Buki is some small settlment about 40 km N to Uman
Ivanki is also small, I think it is village ~20 km S to Buki.
Let me build the address from the beginning: 
"your relatives" lived in Ivan'ki, but there were no post-office.
so the sender directed the letter to the Post in Buki
and explained it is in the "uezd"-district of Uman' which lays in Kiev gubernia
the letter went from Kiev to Uman to Buki and then to Ivan'ka
Josef ASH, Israel