M4B refers to mp4 or m4a files that contain audiobooks, radio plays or podcasts.
iTunes and Apple's iPod have made this format popular. Your M4B files are encoded exclusively in either the lossy AAC or the lossless Apple Lossless.
The peculiarity of M4B over m4a is that the files allow bookmarks to be set so that the listener can return exactly to a previously defined point.
In general, however, M4B files can also contain audio data encoded with other compression methods. However, this is rare.
The contents of the file may also be protected by DRM (Digital Rights Management). Then you need a corresponding, possibly paid license to view the content.
Most audio players can play this format. Whether they support the possibility of bookmarks varies from case to case.
MP3 and MP4 hardware player only in exceptional cases.