What is a Cardano node?
The Cardano node is the core component that underpins the Cardano network. 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's defined by the Ouroboros protocol.
By running a Cardano node, users participate in and contribute to the network. For the Alonzo testnet, nodes will run on a network separate from the Cardano mainnet so that functionality can be tested and verified in a safe sandbox environment.