The Cardano node is the core component that underpins the network and enables you to participate in the Cardano decentralised blockchain. Ultimately, a blockchain network is just a collection of interconnected nodes, all working together to validate transactions and blocks by means of consensus. The definition of consensus for any given network varies, but for the Cardano network it is defined by the Ouroboros protocol. By running a Cardano node, you are participating in and contributing to the network.
This cardano-node is the top level component for the node and aggregates a number of other components from other packages: consensus, ledger and networking, configuration, CLI, as well as logging and monitoring. Note that cardano-node no longer incorporates wallet or explorer functionality. The wallet backend and explorer backend are separate components that run in separate external processes that communicate with the node via local IPC.