High school graduation is a pivotal moment in most people's lives — an important point that signifies a shift in direction.
You see the word pivot in pivotal. That is because when something is pivotal, it is central, and everything related to the topic turns or depends on it. An election can be a pivotal moment in a nation's history if the direction the country takes depends on it. Your belief in reincarnation may be pivotal to how you treat animals. Pivotal means important, but it has the sense of centrality and turning.