Firstly, to understand Spectre, you have to understand Skyfall.
I have posted my "theory of two worlds" before, but here it is in full: Skyfall presents the world as having two distinct halves that exist in a very delicate balance. First, we have our world, or the "light world". It is romanticised, idealised; it is a world where everything is as it should be. But the inverse is a cynical world, a "shadow world" where all the latent evils exist. At the start of Skyfall, there is a very small window where the light and shadow worlds overlap, and Bond exists within that window.
One of my pet hates in popular culture is the typical patriarchal hero who rejects a system that is outdated and replaces society's morality and sense of justice with his own. By the end of the story, society inevitably catches up, vindicating the hero despite the fact that he blatantly abuses the power instilled in him as the defender of the status quo. Bond, however, is unusual in that he is a defender of the status quo, but is never beholden to a conventional system. He operates on the frontier of morality, a space that cannot be defined by law. He is accountable to himself, but has avoided corruption. This is because when he is in the light world, he is a cynical figure, but when he is in the shadow world, he is a romantic. It is this paradox that prevents him from being drawn too deeply into either one.
Skyfall is built on the premise that the window between these two worlds is widening, with the light world becoming increasingly aware of the shadow. The response is to reject the shadow world, to not only separate it from the light, but destroy it entirely. This is impossible because they are two sides of the same coin, so to speak. By the end of Skyfall, the balance is restored, but it comes at a cost: Bond is forced to choose a side. He can no longer be a moderator of the two.
This sets up the conditions for Spectre. With Bond picking a side, a vacuum has emerged, and it must be filled by an entity; that entity is Spectre itself. Where Skyfall attempted to separate the light and the shadow worlds, Spectre attempts to merge them. And so we get the conflict between Bond and Blofeld, with both men constantly trying to redefine the other on their terms. Blofeld credits Bond with his creation, and attempts to destroy his identity. Bond scars Blofeld, and so Blofeld destroys everything that once represented him, forcing him to recommit to making the same choice that he made in Skyfall. In a sense, they are each other's spectres.
I have alluded to the idea of Spectre being a beast that feeds on our own paranoia and insecurity before, and I think that still holds true. They are the physical manifestation of the shadow world gaining a foothold in the light world, attempting to consume it whilst using misdirection to make us believe that we still have freedom. If Skyfall is apocalyptic, then Spectre is the fallout as everything becomes subjectively redefined. The status quo established in Casino Royale and Quantum of Solace is gone, and the world exists in a state of dynamic disequilibrium. Bond is now on a constantly-shifting landscape, but does not have the passive responsibility of maintaining the status quo; Spectre is his discovery that he can wield as much influence as he is subjected to.
Ultimately, I think Spectre will be remembered for how Bond 25 plays out. It feels like the middle chapter in a trilogy (though I suspect that it could run up to Bond 26 and even Bond 27). A lot of the film moves the pieces around on the board, and at the risk of falling into a mixed metaphor, pulling back the curtain and showing us the inner workings of the world following Skyfall.