Political Scientists define Diplomacy as the art of compromise between nations. The topics usually discussed in diplomatic situations usually revolve around trade, war, economics, and maintaining a peaceful coexistence. Nixon's trip to China in 1972 ushered in America's foreign policy of De`tente, a relaxation of international tensions. Americans hoped that the trip would keep the Soviet Union wondering about America's motives. President Nixon favored reopening diplomatic relations with China because it was in America's best interests. China was the world's most populous nation, it was a nuclear power, and China had significant presence in less developed Asian countries. Although the visit was probably more symbolic than practical, it offered a little warmth during the Cold War.
President Richard Nixon's visit to China relates to diplomacy in that it was a major diplomatic initiative. Nixon made the trip in an attempt to set up diplomatic relations with China. The US had not recognized the Chinese communist government since it took over Mainland China in 1949. Nixon wanted to replace this situation with a situation in which diplomacy could happen.
It was easier to open diplomatic relations with China at that point because China was worried about the power of the Soviet Union and it wanted to have a better relationship with the United States. Diplomacy often works this way--Country A (China in this case) can seek better relations with Country B (the US) in order to strengthen it in its competition with Country C (USSR).