The extension tubes let you focus a closer distances than the lens can on its own, they fit between the body and the lens, and include the necessary linkage for the shutter.
The minimum focus distance on the 150 is 1.4 m, and if you want to do a tight head and shoulders portrait, that isn't quite close enough.

There are several extension tubes available designated by their length in mm, the 8 is the shortest, the longest is about 55 mm.
The longer ones are only good for macro use, but tubes like the 8 or 16 will bring your focus distance close enough for a tight head and shoulders portrait.

There is a chart and more info at Hasselbladhistorical; http://www.hasselbladhistorical.eu/P...ls/Extubes.pdf