Literally, a linchpin is the pin that goes through the axle of a wheel to keep it in place, but linchpin can be used to mean an important part of anything, the thing that holds it all together.
The linchpin is something that supports and holds another thing together. It’s the most important part of anything — eating well is the linchpin to staying healthy. A basketball team's most valuable player is the linchpin. The most crucial part of a plan is the linchpin. If something or someone is indispensable, she’s the linchpin. Take away the linchpin and everything falls apart.