The Eastern Front was huge. It stretched for the whole of Russia's European border and involved most of Eastern Europe.
Russia was Serbia's ally. When Austria-Hungary declared war against Serbia, with Germany's support, Russia entered the war to help.
Fighting on the Eastern Front started straight away in 1914 and lasted until Russia signed the Treaty of Brest-Litovsk in March 1918. Russia left the war early because there had been a revolution.
Germany, Austria-Hungary and Bulgaria fought against Romania and the Russian Empire on the Eastern Front.
When fighting on the Eastern Front ended this freed up German soldiers to move to the Western Front to fight in the Spring Offensive. But the arrival of more and more American troops soon meant the Germans were outnumbered. The Russians signed a very harsh treaty with Germany called Brest-Litovsk. The treaty asked Russia to give up lots of land. However the Treaty of Versailles ended up taking back all of the land Germany had gained through Brest Litovsk.