At this conference, held in Teheran in late 1943, the timing of the Allied invasions of Europe and Soviet attacks on Germany were planned. In essence, the broad strokes of how Europe would be taken back from Germany were laid out at this conference.
Coming in to the Teheran conference, Winston Churchill wanted to attack Europe via the Mediterranean. Roosevelt wanted to attack via Normandy and Stalin was on his side. Stalin was particularly eager for the Western Allies to open up a second major front against Germany so as to relieve the pressure on the Soviet Union. When the Normandy invasion was agreed upon and scheduled, Stalin also agreed to launch a major offensive against Germany to coincide with that invasion.
This conference, then, was important in that it got the Allies together and in rough agreement on the broad strategy for retaking Europe from Germany.