A "turning point" is a time or event in history when trends shift and go in a new direction, or when a situation moves from a stalemate or losing situation to progress or a victory. (Or it could be a move from a positive situation to a negative one.)
As an example, let's look at the United States entry into World War I as a turning point in that war. The US entry into World War I was a turning point because it provided money, munition, manpower and morale to help the Allies accomplish victory. Until that point, the war in western Europe had been stalemated. If anything, it looked like Germany was ready to push the Central Powers to victory. But the knowledge that full US financial backing and US troops were on their way gave a huge morale boost to Britain and France as they pressed on to win the war. The arrival of American troops in Europe also helped propel the Allies to final victory.
