what is rapidly changing dimension?

There is no Dimension called Rapidly changing dimension in DWH.  But if you consider overall ODS tables; a rapidly changing dimension is one that holds the transactional data rather than staging data.

There is a Dimension called Monster Dimension.

I hope if you can treat this as a rapidly changing dimension.

A rapidly changing dimension is a result of poor decisions during the requirements analysis and  data modeling stages of the Data Warehousing project. If the data in the dimension table is changing a lot, it is a hint that the design should be revisited.

A dimension is considered to be a rapidly changing dimension if one or more of its attributes changes frequently in many rows. For a rapidly changing dimension, the dimension table can grow very large from the application of numerous Type 2 changes.