In this address he was urging Americans to reduce the amounts of grains that they were using to help Europeans who were starving as a result of famine and recovery from WWII. He also wanted to make sure that his Marshall Plan for European economic recovery did not fall apart.
Television was still pretty new at this point and most Americans still listened to the radio for news and entertainment. Many Americans missed President Truman's television debut, but this first televised speech was the start of a very complex relationship between the White House and the media.
President Truman was the first president to deliver a speech on television, but he was not the first to appear on television. President Franklin Roosevelt was the first president to appear on television. He did so while at the World's Fair in New York City on April 30, 1939.