The "Best Matching Unit" or BMU is a very important concept in the training for a SOM. The BMU is the output neuron that has weight connections to the input neurons that most closely match the current input vector. This neuron (and its "neighborhood") are the neurons that will receive training. This class also tracks the worst distance (of all BMU's). This gives some indication of how well the network is trained, and thus becomes the "error" of the entire network.