Soft release buttons aka "softies" are buttons which create a larger, more protuberant shutter release button on your camera. On Leica rangefinder cameras, there is a threaded hole in the shutter release button for the cable release, and you screw the soft release button into that cable release thread.
43Groups has no affiliation with any of the equipment manufacturers mentioned on this site. Please visit their official websites for more information. All trademarks and brands belong to their respective owners.