MBM is shortened from MultiBitMap which, as the name suggests, is a container for a set of bitmap images. The contained bitmaps are not stored verbatim. Rather, each one is stored with a modified bitmap header with no data compression or with 8-, 12-, 16-, or 24-bit RLE compression.

MBM files are used by most Symbian GUI applications to store their graphical content.
In computer graphics, a bitmap or pixmap is a type of memory organization or image file format used to store digital images. The term bitmap comes from the computer programming terminology, meaning just a map of bits, a spatially mapped array of bits.
Now, along with pixmap, it commonly refers to the similar concept of a spatially mapped array of pixels. Raster images in general may be referred to as bitmaps or pixmaps, whether synthetic or photographic, in files or memory.
In some contexts, the term bitmap implies one bit per pixel, while pixmap is used for images with multiple bits per pixel.
Many graphical user interfaces use bitmaps in their built-in graphics subsystems for example, the Microsoft Windows and OS/2 platforms' GDI subsystem, where the specific format used is the Windows and OS/2 bitmap file format, usually named with the file extension of .BMP (or .DIB for device-independent bitmap). Besides BMP, other file formats that store literal bitmaps include InterLeaved Bitmap (ILBM), Portable Bitmap (PBM), X Bitmap (XBM), and Wireless Application Protocol Bitmap (WBMP). Similarly, most other image file formats, such as JPEG, TIFF, PNG, and GIF, also store bitmap images (as opposed to vector graphics), but they are not usually referred to as bitmaps, since they use compressed formats internally.