What was the significance of the conference held at Yalta and code named "Argonaut?"
The significance of this conference for world history is that it basically split up Europe into Soviet and Western spheres of influence. This was the beginning of the situaton that would develop into the Cold War.
While there were other aspects to the Yalta Conference, the most important one, historically speaking, was the de facto splitting of Europe. This exact issue was not really officially decided upon, but it did come out of the meeting. In the meeting, the Allies officially decided to split Germany up into zones that would each be controlled by one of the Allied powers. This split grew into a de facto split in which Eastern Europe was given to the Soviets and Western Europe was given to the Allies.
There were other aspects to this conference, such as the USSR agreeing to enter the war against Japan later on (which can be said to have led to the creation of a communist North Korea and the ensuing Korean War) and such as the agreement that Soviet citizens taken prisoner would be sent back to the USSR whether they wanted to or not (most were sent straight to prison camps upon returning home). But the most important result of this conference was the split that helped create the Western and Communist blocs that were an important feature of the Cold War.