What is ballpark transform?

From the Proj documentation https://proj.org/glossary.html?

Ballpark transformation

For a transformation between two geographic CRS, a ballpark transformation is a coordinate operation that only takes into account potential difference of axis orders (long-lat vs lat-long), units (degree vs grads) and prime meridian (Greewich vs Paris/Rome/other historic prime meridians). It does not attempt any datum shift, hence the “ballpark” qualifier in its name. Its accuracy is unknown, and could lead in some cases to errors of a few hundreds of metres.

For a transformation between two vertical CRS or a vertical CRS and a geographic CRS, a ballpark transformation only takes into account potential different in units (e.g. metres vs feet). Its accuracy is unknown, and could lead in some cases to errors of a few tens of metres.

Note

The term “Ballpark transformation” is specific to PROJ.

Ballpark is a rather informal term for estimation. QGIS warns you that the transformation probably will be off by a few m and that you should avoid relying on results gained from data transformed in such a way.