Echoing what /u/mc_security has said, the Central Powers 'defeated' Russia through various means, the most obvious of which was sending Lenin and many of his comrades to Russia after the February Revolution, to destabilize the new Republic. Russia was ripe for internal collapse, suffering from economic difficulties, a failure of it's war effort, and poor political leadership. The campaigns of 1915-16 had gutted the Russian Army, so there was little serious resistance. When the Central Powers completed their victory with Operation Faustschlag in March 1918, they never actually made into European Russia proper, but this was enough to force the Bolsheviks to accept Brest-Litovsk.

It's still worth noting that the Eastern Front was a Pyrrhic victory, given that it laid the seeds for the collapse of Austria-Hungary.

Central Powers refers to Germany, Austria, Hungary in World War I. Short answer is that they didn't invade. They fought a series of battles in Prussia and Poland and other places at or near the frontier and after a series of defeats, the Russian army collapsed. In 1917, The new Bolshevik government signed the treaty of Treaty of Brest-Litovsk with the central powers and removed themselves from the war. The central powers then turned their attention to the west and south.