Training a neural network is the process of adjusting its weights and biases so that the network's outputs come to match the desired targets as closely as possible. Starting from an initial set of parameters, the network computes an output via forward propagation, compares that output to the target using a loss (error) function, and then updates the weights and biases based on the resulting error signal, typically via backpropagation combined with a gradient-based optimization step. This iterative fine-tuning of weights and biases from the training data, repeated until the network's performance converges, is what is meant by "training" a neural network.