This part is listed by BMW as BMW part number 11287835422 (11 28 7 835 422) and is described as Hydraulic Belt Tensioner for the power steering and air conditioning belt on 2008-2013 E9X M3 (all cars from 5/2008 production date). Hint: the belt spins counter-clockwise; the early belt spins clockwise.
This is the only the tensioner itself, not the complete assembly, but can be replaced on its own. The tensioner is like a small shock absorber that maintains pressure on the belt. A loose belt will cause undesired performance from the A/C compressor or stiffer steering from lower power steering pump output. It might also lead to the belt slipping from the pulleys or premature belt wear.
This is a Genuine BMW belt tensioner with a two year warranty. Buying a Genuine part will usually be more expensive than an OEM or aftermarket brand but you are assured that it meets BMW's designs for fitment, performance, and longevity. It will also be the very latest design, incorporating any updates or revisions that an aftermarket brand typically will ignore.
BMW Fitments:
3 Series
E90 / E91 / E92 / E93 (06-13) > M3
