What is Modbus?
Modbus is an open messaging protocol: a small, strict set of rules for how one device asks another for data and how the second device answers. Modicon published it in 1979 for its PLCs. It runs over serial lines (RTU and ASCII) and over Ethernet (Modbus TCP).
Strip away every detail and Modbus is just this: a question with a known shape and an answer with a known shape. It doesn't care whether a number is a tank level, a motor speed, or the state of a light. It only defines how the question and the answer travel. Because it does so little, it is easy to implement, easy to debug, and very hard to break in confusing ways.
Modbus is a protocol, not a cable. The same messages can run down a two-wire RS-485 line or cross an Ethernet network alongside email. On serial lines they travel as Modbus RTU (compact binary) or Modbus ASCII (readable text). On Ethernet, Modbus TCP carries the same message behind a short header on the reserved port 502, and lets TCP handle the error checking.
Every conversation has two roles. A client (older manuals say master) asks; a server (slave) answers, and a server never speaks first. What the client can ask for lives in four tables: coils and discrete inputs hold single bits, and holding registers and input registers hold 16-bit words. The numbering of those tables is the thing that trips up every newcomer.
Modicon published Modbus openly, with no license fee, and thousands of manufacturers built it into their meters, drives, inverters, sensors and gateways. It is still free to use, and four decades later it still connects much of the equipment that runs modern industry.