The Syrian Kurdish forces (YPG) are considered extremely valuable in the fight against ISIS. They have previously received support from the United States, although western cooperation with the group has been limited in order to avoid ruffling Erdogan's feathers. But for Russia, upsetting Erdogan is not really an "issue".
In layman's terms, this new alliance could be a fatal blow to Turkey's geopolitical aspirations in Syria (not to mention Russia's de facto no fly zone, thanks largely to a little friend called "the S-400"
The YPG has still not pushed west of the Euphrates, but along with its allies, and with the help of Russian strikes, the SDF are threatening Turkey-backed opposition groups in another key border crossing, Kilis, west of Jarablous.
Losing control of the northern countryside of Aleppo would be a setback for the opposition. Turkey, too, would lose influence.
But Russian President Vladimir Putin seems to be eyeing an even bigger victory. He called on the Assad government and the political wing of the YPG to unite. This has still not happened - at least not officially.
But Syrian Kurdish officials have said they are ready to work with anyone fighting ISIL, and anyone who works for a united, secular and democratic Syria.
Such an alliance would change the battlefield and the balance of power on the ground.