Autocephaly basically means creating the sovereignty of a church. The Pope is the Sovereign head of the Roman Catholic Church for example. He doesn’t answer to anyone above him in the RCC hierarchy. The Russian Orthodox Church does not want the Ukrainian Church to splinter and become its own church with its own head who doesn’t answer to the Russian Orthodox Church.
At least that’s what I make of it. I could be off a bit, not my specialty, but I think there are 9 churches in the Eastern Orthodox group of churches. The Russian Orthodox Church is one of the 9 and I think there are 4 Greek Orthodox churches in the group but don’t hold me to that. It seems that the Ukranian church wants to become #10.
There is already an autonomous Ukrainian Orthodox Church, whose head sits in the Assembly of the Russian Orthodox Church (not vice versa). Patriarch Bartholomew has now, in violation of church canons, elevated two separate schismatic sects, lead by deposed clerics, to be a new autocephalous Orthodox Church of Ukraine. He himself acknowledged their illegitimate status but two years ago, but since then "something's changed." Whether these two sects will now manage to work together has yet to be seen.