Complex sentence: The sign was a reference to the NFL team with the same name, which was a league powerhouse at the time, having appeared the NFL championship game in the three seasons prior to the short being released, winning the title in 1952 and 1953.
Split into simpler sentences: The sign was a reference to the NFL team with the same name, which was a league powerhouse at the time. The NFL Lions had appeared the league championship game in the three seasons prior to the short being released, winning the title in 1952 and 1953.