Quantum field theory (QFT) is a theoretical framework that describes the behavior of fundamental particles and their interactions by incorporating both quantum mechanics and special relativity. At the most foundational level, QFT describes reality in terms of quantum fields.
Quantum fields are mathematical constructs that pervade all of spacetime. They are associated with different types of particles and represent the underlying "stuff" from which particles emerge. In QFT, particles are considered as excitations or quanta of their respective quantum fields.
According to the current understanding in particle physics, the most foundational level of reality described by QFT is the set of quantum fields that correspond to the known fundamental particles and their interactions. These particles include quarks, leptons (such as electrons and neutrinos), and gauge bosons (such as photons and W and Z bosons), among others.
The interactions between these quantum fields are mediated by the exchange of other particles, such as virtual photons or gluons. The behavior of these fields and their interactions are governed by specific equations, such as the Dirac equation or the Klein-Gordon equation, which are part of the formalism of QFT.
It is important to note that our current understanding of physics is not complete, and there are open questions and ongoing research to further deepen our understanding of the most foundational level of reality. The quest for a more fundamental theory, such as a theory of quantum gravity that unifies general relativity with quantum mechanics, is an active area of research in theoretical physics.