"Manufacturing execution system" is a heavy phrase for a fairly simple idea. This is a plain, jargon-free explanation of what an MES is and what it does, for anyone who has met the term and wants it explained clearly.
The short answer
A manufacturing execution system, MES for short, is software that tracks and manages production on the factory floor as it happens. It is the system that knows, in real time, what is being made, on which machine, by whom, and how it is going. If a planning system is concerned with what should be made, an MES is concerned with what is being made right now.
Breaking down the name
The name is more helpful than it first looks. Manufacturing, because it is about making things. Execution, because it deals with the doing, the actual running of work on the floor, not the planning of it. System, because it is the software that ties that together. An MES is, quite literally, the system for the execution of manufacturing.
What it does, in everyday terms
Picture a factory floor without an MES. Operators work from printed job sheets. Someone walks the floor to see how orders are progressing. Output, scrap, and downtime are written down and totalled at the end of the shift. By the time the numbers are known, the shift is over and nothing can be changed.
An MES replaces that with something live. It tells each operator what to run and shows the correct instructions at the station. It records what happens, started, paused, finished, good parts, bad parts, as it happens. It collects data, from operators and often from the machines directly. And it turns that data into a current picture of the floor: where every order stands, how fast things are running, where time is being lost. The shift-end surprise becomes a live readout.
Where it sits among other systems
An MES is usually described as the middle layer of a factory's software. Above it is the planning system, the ERP, which decides what to make and what it costs. Below it are the machines. The MES sits between them: it takes the planned orders and manages and tracks their execution on the floor. That middle position is what an MES is for.
Does every factory have one?
No, and that is an important point. A dedicated MES is most common in larger, faster, or highly automated plants, and in regulated industries that need detailed production records. Many smaller and mid-sized manufacturers do not run a separate MES at all. Instead they use the shop-floor tracking built into their manufacturing ERP, which for their scale covers what an MES would do. Whether a plant needs a dedicated MES depends on its size, speed, and industry, not on whether MES software exists.
The one-sentence version
A manufacturing execution system is the software that watches and manages production on the shop floor in real time, so a factory knows what is happening while it is happening instead of finding out when the shift ends.