It means installing the electrical equipment (transformers, switchgear, motors, motor control centres, protection, metering and monitoring cubicles, lighting, instruments and controls) pulling the cables, laying cable trays and conduit and connecting the electrical wiring of a project.
Normally erection and commissioning are separate functions, mainly because they require different expertise and therefore different staff. They might be grouped together if the technical requirements of running and testing the particular installation are quite simple.