What it says on the tin. OTL, the Soviets were notoriously caught with their pants down when the Nazi's launched Barbarossa, and the Nazi's were able to penetrate deep into Soviet territory (before running out of steam and being turned back, we know the story). The Red Army, despite an on-paper advantage, was dispersed and didn't have adequate transportation, and Stalin had completely misjudged where the invasion would take place.
What if the Soviets had anticipated Barbarossa and taken further steps to prepare? In August of 1940, the British caught wind that Hitler had approved the operation had attempted to warn the Soviets, but Stalin refused to believe them. Now, let's say that he instead takes their warnings as, if not fact, as a matter of serious consideration, and instructs his (remaining, this is post-Great Purge mind you) generals to begin preparations for a German invasion sometime in early 1941 (furthermore, let's say Richard Sorge still gives him the exact date of the invasion).
Hitler, being Hitler, continues with the invasion even if the German realize the Soviets have caught wind of his plans, and Barbarossa is launched on schedule. How does it go? How does this affect the rest of the war?