What happened in Russia that led Germany to change its war strategy in World War I?

What happened was the Russian Revolution. In 1917, angry at the deprivations caused by the war, Russian people in Petrograd, the imperial capital, took to the streets in protest. When Russian soldiers joined them, the position of Tsar Nicholas II became untenable, and he abdicated. He was replaced by a provisional government that disappointed the people by keeping Russia in the war. Continued anger at the war effort led to another stage of the revolution led by Bolsheviks, Communist insurgents under the leadership of Vladimir Lenin (with some clandestine assistance from Germany) overthrew the provisional government and eventually gained control. 

This affected German strategy because the Bolsheviks quickly sought a treaty to get out of the war. The Treaty of Brest-Litovsk, concluded in 1918, concluded Russia's involvement while giving vast swaths of land to Germany. For the Germans, who had been fighting a two-front war, this treaty allowed them to focus on the Western Front. Unfortunately for them, their offensive in the spring was driven back by the Allies, recently reinforced by the American Expeditionary Force, and the German war effort collapsed. But the Russian Revolution briefly allowed the Germans to put their full effort into fighting in western Europe.