What does nb_epoch in neural network stands for?

i'm currently beginning to discover Keras library for deap learning, it seems that in the training phase a centain number of epoch is chosen, but i don't know on which assumption is this choice based on.

In the Mnist dataset the number of epochs chosen is 4 :

model.fit(X_train, Y_train,
          batch_size=128, nb_epoch=4,
          show_accuracy=True, verbose=1,
          validation_data=(X_test, Y_test))

Could someone tell me why and how do we choose a correct number of epochs ?