This is a function we need to specify to a the keras.compile function to measure the output accuracy of an NN. https://keras.io/metrics/
This has nothing to do with how the NN works but rather to measure the final output accuracy.
To complete our NN, we finally use this accuracy as follows
model.compile(Adam(lr=.0001),loss="sparse_categorical_crossentropy",metrics=['accuracy']) # this completes our first neural network
These are helper function which are optional to use in case we need to initialize our NN weights with some specific values instead of total random values. https://keras.io/initializers/
model.add(Dense(64, kernel_initializer='random_uniform', bias_initializer='zeros'))