Russia is still under sanctions, so it's probably trading stuff it has to spare - food, fertilizer, and oil/gas, meaning this is something that's beneficial for them.
North Korea is probably handing over some of their older weapons (so they don't have to maintain or dispose of them) as well as some of their newer weapons so that they can get feedback on how those weapons perform in real life combat, so this is something that is wish beneficial for them.
Since Russia and North Korea share a border, this trade seems almost inevitable.