The Russian or Eastern front involved combat on a very wide front, from the Baltic to the Black Sea. The principal combatants were the Russian Empire versus the Central Powers, Austria-Hungary and Germany. Other minor states took part including Romania, Bulgaria and the Ottoman Empire.
Fighting on the Eastern Front did not bog down into a static war of attrition and trench warfare, as it did on the Western Front. There was considerable movement back and forth as both sides made deep thrusts into each other territories. The Russians launched offensives into Austria-Hungary and Prussia. They achieved notable successes against the Austrians, but overall the Russian armies were poorly led and under equipped.
Eventually the tied turned against the Russians and they were pushed back, first out of Poland (then a Russian possession) and then into the Ukraine. Discontent with the war and their supreme commander, the Czar, grew among the Russian people. As defeat followed defeat, the military became dispirited and the population moved towards revolution. The failure of the Russian war effort was a significant cause of the Revolution which brought the Bolsheviks to power and ended the Russian monarchy.
In 1917 he Russians then made a separate peace with Germany and Austria known as the Treaty of Brest-Litovsk, which left large areas of Russian territory under occupation. It also led to the secession and independence of significant Russian territories such as Poland, Finland, and for a time the Ukraine.
More importantly, the Russian collapse freed up large numbers of German and Austrian troops that could now be deployed on other fronts. The Austrians then turned their attention to the Italian front while the Germans massed their troops on the French front in an effort to break through to Paris.