Nodes are how Bitcoin has no boss: each one enforces the rules itself and rejects anything invalid, no matter who mined it. Thousands of independent nodes make rule changes impossible to impose from above.
Running one takes any modern computer (or a Raspberry Pi with Umbrel/Start9) and ~600 GB of disk. Point your wallet at it and your balance queries stop leaking to third-party servers — the privacy upgrade every desktop wallet review on this site mentions. See
Bitcoin Core.