The second front refers to Stalin's wish for the western allies open another front in Europe in 1942 against the germans to take the pressure of the Russians on the Eastern Front. (An invasion of France)
He wanted the Allies to invade mainland Europe as soon as possible to force the Germans to withdraw some of their troops from the Eastern Front. During this period of the war the Germans were pushing far into Southern Russia heading towards Stalingrad.
Roosevelt was keen and willing to launch an invasion in 42 or 43 but Churchill and the british did not think it would be successful. The allies did not have any landing craft, they did not have air superiority and the British Army was still under prepared and equipped to face the german army in Europe again. While the British had a powerful naval and Airforce more would still be needed. The American's were still in the process of putting an army together, their industrial might was only just beinging and they had no troops with any combat experiance at all. Their navy was heavily involved in the Pacific and new units would be needed for the European Front. Their airforce had old and outdated planes which would have been no match for the German airforce which was still a powerful force at this stage of the war.
The British Command including Churchill were still haunted by the mass slaughter of the First world war, and as such wanted to ensure they had enough planes, tanks, men and ships to increse the chances of any invasion being successful, during 1942 and 1943 the allies were still a long way from this and a rift started between the western allies on this issue. It was only with great reluctance that the British actually agreed to an invasion in 1944 in the end.