A computer that is part of a network. It is essentially a connection point among network devices such as routers, printers, or switches. This allows the sending of data from one endpoint to the other.
Blockchains are a network of nodes.
This is similar to the overall Internet which can be thought as a web of nodes connected by common protocols.
Financial systems are mostly networks these days with money being essentially ledgers. Because of this, monetary systems are, at least in part, a series of nodes strung together. This is the result of the expansion of the digital world.