A node is simply a computer that participates in a blockchain network by keeping a copy of the shared ledger and following the network rules. Instead of one central server, a blockchain is maintained by many nodes spread around the world, each holding the same history of transactions.
Nodes do important work. They receive new transactions, check them against the rules, and pass along valid ones to other nodes. When they all agree on which transactions are legitimate, the network stays consistent. Because every node can verify things independently, no single participant has to be trusted.
This distribution is the heart of decentralization. The more independent nodes there are, the harder it becomes for any one party to control or corrupt the record. Running a node is open to anyone with suitable hardware, which is part of what keeps many networks open and resilient.