https://www.bloomberg.com/news/articles/2019-12-21/gazprom-ukraine-agree-on-transit-volumes-settle-legal-issues

Gazprom is a Russian natural gas company who is Europe's largest and (mostly) cheapest supplier of natural gas. The reason Ukraine is involved is because they physically have the main pipelines running through their country, from Russia into Europe. Not too long ago Russia had been planning to completely cease the delivery of natural gas to Europe via Ukraine, instead opting for alternative pipelines they began constructing under Baltic sea, dubbed Nord Stream 2. This pipeline has been opposed by the US and sanctions against Russia forced their sub-contractors to cease construction of the pipeline, for now.

This new deal between Russia and Ukraine, which guarantees the continued use of these pipelines through the end of 2024, is important for multiple reasons:

1. It ensures (at least through the end of 2024) that Europe does not have a massive shortage of natural gas, which would result in a huge energy crisis.
2. As a part of this deal, Russia and Ukraine have both mutually agreed to drop various lawsuits held against each other relating to the pipelines. This alone is seen as a landmark move that could pave the way for a friendlier future between Russia and Ukraine.