Agents of SHIELD season 6 just brought back the Mind Prison seen in Captain Marvel. Captain Marvel revealed that the Kree possess the technology to project a person's mind on to a realm they called the Astralscape, where they communed mentally with the Supreme Intelligence. In a third act twist, the Kree even attempted to use an Astralscape as a prison for Carol Danvers, with the Supreme Intelligence taking full command of this psychic reality.
Unlike most MCU movies which tend to mostly ignore Marvel TV entirely, Captain Marvel had an unusually close relationship with Agents of SHIELD. The film went to remarkable lengths to avoid contradicting what the ABC series had introduced, especially when it came to Kree technology (the species had been important since season 1). The Astralscape itself had appeared in Agents of SHIELD season 5, when the Confederacy used a version of it in an attempt to restrain Quake.
The latest episode of Agents of SHIELD season 6, "Inescapable," saw the most creative use of the Astralscape to date. The Chronicoms imprisoned Fitz and Simmons inside a shared Astralscape using a cerebral fusion machine. They allowed the two an unprecedented amount of control over the environment, hoping that the two human scientists could use it to crack time travel. What they failed to consider, however, was just how fractured Fitz and Simmons' psyches are. Both have been repressing their emotions for years, and that repression meant the Astralscape became a distinctly dangerous place to be.
This is what Agents of SHIELD has always been best at: lifting an idea from the movies, and then developing it to an unprecedented degree. The Astralscape became a shared representation of Fitz and Simmons' fractured psyches. At one point, Simmons regressed to a child who was unable to cope with the world around her and just wanted to retreat into her bed; the music box her father had encouraged her to bury her pain in became a symbol of her repression, containing all her years of pain and fear, unleashing a monster when it was opened. In narrative terms, the Astralscape became a perfect way to "show, not tell" - to allow both Fitz and Simmons to explain the events of season 5, without excessive infodumps. Interestingly, one line of dialogue suggested there's no relationship between time spent in the Astralscape and the real world; apparently Fitz and Simmons were only in this Astralscape for five minutes, even though they clearly experienced it as several hours at least.
Meanwhile, "Inescapable" subtly shone a new light upon the heroism of both Captain Marvel and Quake. According to the Chronicoms in Agents of SHIELD, any attempt to escape an Astralscape potentially induces madness. But both superheroes pulled it off by sheer force of will, confirming just how powerful they really are.