Hi,
Do you expect us to create a training material especially for you? Please read more careful and when you have more specific questions than "What that means , why it is needed and how it operates" come back
About the multiplexing, this is a term that Oracle uses for keeping multiple identical copies of the same file, when these files are of particular importance and their lost would possibly cause data loss ( i.e redo log files and control file)
The term MULTIPLEXING is used to distinguish between this mechanism, supported by Oracle and operating at file level and the mirroring - supported by the OS or the storage and operating on disk level.