Ground coriander is referring to the seeds. It’s grinding the dried seeds which you can buy pre-ground or grind yourself in a spice grinder or mortar and pestle.
This is not to be confused with cilantro which is the fresh green leaves of the same plant.
If you’re confused it’s probably a regional difference of dialect. I’ve heard that outside of North America coriander can often refer to the leaves of the plant, but here in North America coriander is the seeds and cilantro is the leaves of the Coriandrum sativum plant.