It allowed Hitler to invade Poland without Soviet interference.
The German-Soviet Non-Aggression Pact, signed in August, 1939, was a promise by both countries that they would take no military action against one another for the next 10 years. For Stalin and the USSR, that would give them time to build up the Soviet military without the threat of a war with Germany. For Hitler and Germany, it allowed the takeover of Poland without fearing a Soviet response from the East.
Later on, as we know, Hitler violated the pact, in 1941, when he began Operation Barbarossa, a massive incursion of German troops into Soviet territory.
