The official Tenet trailer confirms what the movie's title means - it’s a codeword. As for Tenet’s central mystery, well, that’s another story as the trailer teases a threat that’s “much worse” than a World War III nuclear holocaust. The key to the unlocking the story's central mystery seemingly correlates with the use of “Tenet” in social situations.
Tenet marks the 11th movie for Christopher Nolan, who has not only made iconic actions films (The Dark Knight trilogy, Dunkirk) but also complex psychological thrillers that focus heavily on memory and cognitive dissonance. The 2000 neo-noir Memento follows a man with short-term memory, and the 2010 hit Inception is fundamentally about the human subconscious. In Tenet, John David Washington’s character - maybe a soldier, maybe a spy - proves to be a valuable asset for underground organizations, as he passes tests with flying colors and tries to learn what exactly he’s needed for. It turns out that he’s trying to protect planet Earth, but from whom, and why? The work becomes especially challenging for Washington’s character when he learns that he can’t necessarily “understand” the world he’s living in, but rather must “feel it.”
Exactly halfway through the Tenet trailer, Martin Donovan’s character explains what “Tenet” means. It’s a cryptic reveal, as he tells Washington that “it will open the right doors… some of the wrong ones, too." The reveal takes place aboard a ship called the “Magne Viking,” where Washington’s character seems to be training for a mission, and also appears to be slightly unnerved by the lack of mission information. When Donovan references “the right doors,” Michael Caine’s suit-and-tie character appears in a restaurant setting, while “the wrong ones” dialogue plays over a shot of a smirking Kenneth Branagh on board a ship. At this point, it seems that context is key for how and where one speaks the word "Tenet."
Directly after the “Tenet” explanation, Dimple Kapadia tells Washington “You have to look at the world in a new way.” On the surface, this line appears to set up the trailer’s climactic car chase, in which time reverses - Washington sees the world in a new way. But it’s possible that Kapadia’s line is merely narrative misdirection for what’s really going on in Tenet. For example, she might be telling Washington that he must look at the word Tenet in a different way, but the trailer editing curiously makes it impossible to read the character’s lips. Meaning, Kapadia only appears while saying “new way.” With a Nolan film, anything is possible. Washington's Tenet character will most likely have to look at the world differently as new information emerges, but he'll also have to pinpoint the exact nature of what the word "Tenet" implies.
With Tenet, Nolan seems to be innovating the “MacGuffin” concept. For example, Inception’s story is based around the title word itself (planting memories into one’s subconscious); however, the actual MacGuffin is a totem, an object that helps Leonardo DiCaprio’s Dom Cobb decipher if he’s dreaming or living in the real world. For Tenet, the title isn’t necessarily the premise but rather an conceptual MacGuffin. It’s just unclear what the word derives from - a physical location, a metaphysical realm, or secret information. Considering what Washington learns and experiences in the Tenet trailer, there’s no established set of rules associated with the word.