von Neumann algebra describes subsystems in quantum mechanics and quantum field theory. It captures quantum information theoretic aspects of the subsystem, e.g. entanglement entropy. I'll do a light introduction to this subject; I'll start by introducing a theorem called Reeh-Schlieder theorem, which says all field variables in any one region of spacetime are entangled with variables in other regions. I'll talk about some of its implications, and then conclude the talk by defining what von Neumann algebra is. The main reference is https://arxiv.org/abs/1803.04993.