From https://learn.microsoft.com/en-us/dotnet/standard/io/memory-mapped-files -- "A memory-mapped file contains the contents of a file in virtual memory."
Memory-mapped files are backed by either an actual file on disk or the system page file.
I'm guessing that you are asking about non-persisted memory mapped files. So in that context, the phrase is referring to virtual memory that will be back by the system page file.