The Inhumans are a race of beings in the Marvel Universe who were created via alien genetic manipulation centuries ago. They also use a semi-mystical gas called Terrigen to go through a process called Terrigenesis that gives them crazy powers, and often leaves them looking more, well, inhuman.
X-Men, and mutants, are a genetic offshoot from baseline humanity, with wild and crazy superpowers that typically manifest at puberty. Their strange and often frightening powers and differences have led to the world fearing and hating them, and the X-Men fight to get to a world where mutants and humans can co-exist.
Some time ago however, Marvel decided to really push hard for the Inhumans, and cut back on X-Men titles significantly. This was because Marvel doesn't hold the film rights to mutants and the X-Men, which they sold to Fox decades ago. The fact that Fox keeps making financially successful movies from them certainly doesn't help.
Marvel wants in on that action, so certainly in the movies and TV shows of the Marvel Cinematic Universe they have been really pushing the Inhumans as a version of the mutant metaphor for audiences. In the comics, this didn't work out and things are reverting to how they were before, but the shows and films still want Inhumans to happen.
But can they really? Are Inhumans similar enough to mutants to be an adequate replacement? We certainly don't think so.