A fulcrum is the supporting point of a lever. The first fulcrum you encountered was probably on the playground — right underneath the see-saw.
Back in the 17th century, a fulcrum referred to any general prop or support. Today it’s taken on more scientific connotations, thanks to physics class. However, people still use fulcrum to describe something that plays an essential role or serves as the center of an activity or situation, such as "the fulcrum of the debate." That dad who organizes everybody’s playdates? He's the fulcrum of the playground social circle.