'Daemon' is the general term for a piece of software running in the background. In Monero, the Daemon is started through the 'monerod' program. If you run the Daemon locally, you are running a local node. If the Daemon is running on another device it's a remote node. A wallet, like the CLI or the GUI, needs to connect to a Daemon (local or remote) to relay transactions to the network.
It's possible to send commands to the Daemon directly or through the RPC interface. See the Daemon RPC guide, which contains a detailed explanation (with examples) of the available RPC calls. For more detailed and technical information about the Daemon, see the Monerodocs reference at the bottom of this page.
2. 'Daemon' entry on Wikipedia
3. Monerod-tagged questions on StackExchange