A focal point was the center of attention or activity.
In 1930, Edith Keeler was the focal point of history that James T. Kirk and Spock had been searching for. Leonard McCoy had prevented her death after traveling through the Guardian of Forever. Keeler's influence on politics had led to the United States of America delaying their entry into World War II, which Nazi Germany won. (TOS: "The City on the Edge of Forever")