Through the Syrian Civil War, Israel has maintained nominal neutrality, while at the same time launching repeated airstrikes against Syrian military targets across the nation. Russia has, so far, looked the other way when this happened, and Israel wants to keep it that way.
So while Israel and Russia already have a “coordination” deal which is aimed at keeping Russian planes from going after Israeli jets during their attack runs, the addition of Russian S-300 missiles to the country has added another source of concern, and has Israel scrambling to revise the coordination deal.
The S-300 missile defense system is hugely advanced and covers a broad amount of territory, allowing Russia to essentially impose a no-fly zone over a vast area if it so chooses. The deployment is primarily aimed at blunting US threats to attack Syrian military bases with Russian soldiers on them, but Israel clearly could find itself in the line of fire too if a deal isn’t reached.
Israel’s attacks have largely been irrelevant to the Civil War itself, so Russia has mostly ignored them, though they have warned Israel away when warplanes have gotten close to the Russian naval base at Tartus. With Russia both increasingly on edge about being attacked in Syria, and having enhanced capabilities to respond to perceived threats, Israel likely will be locked out of Syria in the absence of such a deal.