While fans have long been divided on ABC's Marvel's Agents of S.H.I.E.L.D., Marvel Studios head honcho, Kevin Feige, has shown a consistent willingness to let the show introduce new concepts to the MCU as a whole. The race known as Inhumans appeared in Marvel comics as far back as 1965, and their story (in a nutshell) is as follows: millennia ago, an alien race called the Kree performed genetic experiments on primitive homo sapiens, in an attempt to breed soldiers to use against their enemies, the Skrulls (known in the MCU as the Chitauri). The experiments were abandoned, and the Inhuman society developed on Earth separately from the rest of the population, in the city of Attilan. When exposed to mutagenic minerals called Terrigen Crystals—often in the form of a mist—Inhumans mutate, and are granted widely varying powers and abilities.
The MCU's version of this race debuted in season 2 of Agents of S.H.I.E.L.D., and Inhumans (and the public's response to them) has remained a key element of the show's storylines ever since. Female lead Daisy Johnson (Chloe Bennet) was revealed to be Inhuman midway through season 2, and underwent the "Terrigenesis" process, giving her the ability to vibrate molecules on scales large and small.
Fans of the show — which has always struggled in the ratings department — appreciated that it was given the opportunity to do this narrative heavy lifting, and relieve the future Inhumans project of a huge burden in terms of exposition. The show also featured the first appearance of the Kree, which will almost certainly figure into the upcoming project as well.