The Bolsheviks, under the leadership of Lenin, had always been bitterly opposed to Russia's participation in World War I. They regarded it as a rich man's war, a conflict between the crowned heads of Europe that had nothing to do with the working classes.
Throughout the conflict, the Bolsheviks tirelessly agitated for Russia to pull out of the war. As the WWI dragged on, without any discernible benefits accruing to the Russian people, it became less and less popular, and increasing numbers of Russians began to support the Bolsheviks, the most consistently anti-war party in Tsarist Russia.
When the Bolsheviks seized power in the insurrection of October 1917, they immediately set about extricating Russia from the fighting. The following year, they signed a pact with the Germans, the Treaty of Brest-Litovsk, that formally ended Russia's participation in World War I.
Under the humiliating terms of this agreement, Russia was required to cede vast swathes of territory to the Germans, including the Baltic States. For good measure, Russia was required to pay Germany a staggering sum of six billion good marks in reparations.
On the face of it, the Treaty of Brest-Litovsk was a crushing victory for the Germans. In due course, however, it became clear that it was more trouble than it was worth. For one thing, the Germans needed to control and defend the vast new territories that they'd acquired under the Treaty. This meant deploying large numbers of soldiers who would otherwise have fought at the front.
In turn, this meant that the Germans had fewer troops available than originally envisaged for their spring offensive, which ultimately ended in failure. Although the Germans gained new territory during the offensive, they found it as hard to defend as that which they had acquired from Russia. Dangerously overstretched on two fronts, the German Army was in a perilous position.
As a consequence, the Central Powers as a whole were greatly weakened. One could say, then, that the Russian withdrawal from the war, far from helping the Central Powers, actually contributed in no small way to their eventual defeat.