It’s theories about how spheres of influence work, that Russia has a right to take over Ukraine, or at least override it politically. Very much similar to Kissinger’s Great power politics in the days of the USSR. It’s somewhat different than the Russian right which is their divine right to Empire, over the Ukrainians and the Poles and Slavic countries in general. The ideologues Ivan Ilyin and Karl Schmidt influence that part. But notably, both parts believe that there’s a place that Russia must dominate in Europe, and that other great powers must not interfere there. Leftists also are influenced by these theories, especially when they remember the reaches of the Iron Curtain far into Central Europe. The USSR had a history of intervening into the politics of its satellite states. Notably in Hungary when there were democratic protests, they sent in tanks to quell the uprising. This theory echoes in 2014 when the Ukrainians changed their government, and Russia invades and annexes Crimea. Many on the far left and far right see the massive protests as creeping American influence that does not belong in the region. They fear NATO expansion as it is a threat to Russia; In the west it’s Russia itself, and in Russia, it’s Russian greatness in Empire.