In computer science, the term "node" simply means a device that plays a role in a larger network.
In the context of cryptocurrencies and the blockchain, a node is one of the computers that runs the blockchain software to validate and store the entire history of transactions on the network.
How do nodes work?
Blockchains are distributed ledgers that store the entire history of transactions on a given network. It is a sequence (chain) of records (blocks) of transactions that have been recognized as legitimate by all members of the network.
In case you are wondering where the blockchain is located - it is in the nodes. Each node contains an identical version of the transactions.
When a new group of transactions (a block) appears in the blockchain, it is transferred from node to node, so each of them can update their own database in the same way.
You may have heard that a blockchain network is "distributed" and "peer-to-peer." This is because nodes are responsible for maintaining the correct database of past transactions in a distributed manner and validating the transactions of the other nodes in the network.
Who can run a node?
In traditional finance, payment networks like Visa or Paypal are run by a central administration.
With most cryptocurrencies, nodes on a blockchain do not rely on validation from above, as they check and verify each other through a consensus mechanism.
Thus, anyone can set up a node by downloading the blockchain's software to their computer, anywhere in the world.