For most of the 20th century, a significant portion of academic philosophy operated under the assumption that the big metaphysical questions, what is reality made of? what kinds of things really exist?, were either unanswerable or just badly posed. The logical positivists declared that metaphysical statements were meaningless (not false, just empty). The linguistic turn suggested philosophy should analyze language rather than describe reality. Continental philosophy after Heidegger focused on human existence, history, and meaning rather than mind-independent reality. The result was a long period in which doing serious metaphysics was considered a bit naive, a bit gauche, something the well-educated philosopher had moved beyond.
Then, in the early 2000s, something shifted. A loose constellation of philosophers, Graham Harman, Quentin Meillassoux, Levi Bryant, Ian Bogost, and others, started asking the big questions again, without apologizing for it. The movement they produced goes by several names: Speculative Realism (the broader umbrella), Object-Oriented Ontology (OOO, Harman's specific version), and sometimes just the new metaphysics.
The philosophical context for understanding what they were reacting against is important. The dominant tradition in both analytic and continental philosophy had been some form of what Meillassoux calls correlationism: the view that we can only ever speak of the correlation between thought and world, never of the world as it is in itself, independently of our access to it. Kant is the paradigm correlationalist: we can know phenomena (things as they appear to us, structured by our cognitive apparatus) but not noumena (things in themselves). 20th century philosophy largely followed Kant here, in various idioms: we can only speak of the world as it is for us, as it is constructed by our language and concepts, as it is disclosed in our practices. The idea of a mind-independent reality that we can meaningfully describe was treated with deep suspicion.
Speculative Realism is, at its most basic, the rejection of correlationism. Reality exists independently of our access to it. Things are real, and they have properties, and enter into relations, whether or not any mind is attending to them. This sounds obvious, but working out what it means philosophically, without falling back into naive pre-Kantian realism that ignores everything Kant showed about the role of our cognitive apparatus in structuring experience, turns out to be genuinely difficult.
Graham Harman's version, Object-Oriented Ontology, is the most developed and probably the most philosophically provocative. His central move is to take seriously the ontological status of objects, not just the objects of everyday experience (tables, rocks, organisms) but all structured entities at every scale, from quarks to galaxies, from corporations to ecosystems to fictional characters. Harman argues that objects are real, that they have a genuine interior that is never fully expressed in their relations with other things, and that the fascination with human subjects and human experience that has dominated modern philosophy has systematically obscured this.
Quick reflection
Before reading further: do you think a rock has an interior? Not in the sense of a hollow space, but in the sense of some genuine being, some way of being what it is that exceeds any description from outside it? Most people immediately say no — rocks are just rocks, described completely by their physical properties. Is that intuition the result of careful thinking, or just a habit? And does it matter for anything practical?