The document discusses neural networks and how they can be viewed as functions. It describes how neural networks take input data and produce output predictions or classifications. The document outlines how neural networks have a layered structure where each layer is a function, and how the layers are composed together. It explains that neurons are the basic units of computation in each layer and how they operate. The document also discusses how neural network training works by optimizing the weights and biases in each layer to minimize error, and how matrix operations in neural networks can benefit from parallel processing on GPUs.