In short
MES is the software layer that tracks, records and manages the execution of work orders on the shop floor in real time.
MES bridges the planning layer (ERP/MRP) and the machine and control layer (PLC, SCADA). It makes visible, in real time, how the plan is actually executed on the floor.
Typical MES functions include work order tracking, machine status and downtime recording, operator inputs, quality data collection, batch/lot traceability and OEE calculation.
A well-designed MES collects shop-floor data directly, reducing manual entry, and feeds it back into planning.