MES (manufacturing execution system)
An MES collects data on how production is running on the shop floor: machine states, piece counters, downtime and operation confirmations. It shows what is happening at a work cell right now instead of reconstructing yesterday's shift from paper notes.
The data arrives along two paths. Newer machining centres and machines hand it over through OPC UA from Siemens or Beckhoff controllers: cycle start and stop, piece counter, alarms, process parameters. On older equipment a sensor on the tool movement is enough, or a terminal where the operator reports the start of an operation and picks a downtime reason from a list.
Out of that comes a live picture of the floor: work in progress, OEE per cell, a history of changeovers, and product genealogy, meaning a record of which material and which machine a given batch came from. When a complaint arrives, that narrows the investigation to one batch instead of a whole month.
An MES does not replace the ERP. The ERP says what has to be made and by when, the MES records how it went and sends back measured times, material consumption and scrap. Order cost then comes from measurement rather than from a standard entered years ago.
The hard part is rarely connecting the machines. It starts with deciding who confirms an operation and which list of downtime reasons they use, because that decides whether the reports can be read at all. The signal that an MES is needed is the question "how much have we made" being answered by walking the floor.