I would summarize it as "pretension without payoff."

The term "mystery box" comes from a J.J. Abrams TED talk the filmmaker delivered in 2007. In his speech, Abrams argued that all stories are essentially a series of questions and mysteries leading the audience from beginning to end. Somewhat ironically, he uses the original Star Wars film as an example - "who is the woman fiddling with R2-D2?" "Who is Obi-Wan Kenobi?" But Abrams also brings onstage a real, physical magic box from his childhood that was never opened. The director uses this nostalgic toy to argue that the creation of mystery and the sense of potential is sometimes the most exciting and engaging part of a story. Young Abrams didn't open his $15 magic box because the thought of what might be inside was more interesting than actually ripping the thing open and finding out.

Abrams has applied this concept to his movies and TV shows over the years, but with mixed results. An incredibly effective example of the mystery box at work can be found in Cloverfield, which Abrams produced. The mystery box here is the origin of the titular monster and how it came to arrive on Earth. Throughout the film, that mystery intrigues and fascinates, but the box is never opened. In his TED talk, Abrams stated that the true purpose of the mystery box is to elicit character development and emotion. Where Cloverfield succeeds is by capturing its audience with the mystery of the monster, but never needing to open the box in order to develop the characters. Where Clover came from has no bearing on the plight of the protagonists or the impact of the monster arriving in New York City.

Examples of less effective mystery boxes can be found in Lost. Listing all of Lost's boxes would take far too long, but some of the biggest include the importance of Walt, the meaning of the numbers, and the true nature of the island, and these three mystery boxes highlight three different pitfalls with Abrams' theory. Lost's Walt mystery box wasn't just left unopened but abandoned altogether, proving that a box needs to stay present in the story in order to be effective. Lost's strange recurring sequence of numbers proved that, sometimes, a mystery does need to be opened in order to satisfy viewers. And lastly, the island was so integral in the development of characters such as Locke, Jack and Hurley, that not opening the box on its true purpose hindered character development, rather than facilitating it.

https://screenrant.com/star-wars-rise-skywalker-abrams-mystery-box-bad/