HarfBuzz is an OpenType text shaping engine. A shaping engine is necessary to properly show the shape of characters that are part of a font. HarfBuzz is the shaping engine that is used for instance by Firefox, Chromium and Android.
As changes to Unicode do not only involve changes to characters but also to the shaping of characters, it is important to have a shaping engine that is up to date and works well with the fonts provided. HarfBuzz is being worked on by strong players in the Open Source world, what the Wikimedia Foundation can do is understand the importance of the stack and work on those parts where it will make an obvious difference; making sure that the scripts, the fonts for its languages are well supported and stimulate the development of tools like HarfBuzz.
More detailed information on HarfBuzz and its current issues for several scripts.