What does model_dir refers to in TensorFlow?

# Build 3 layer DNN with 10, 20, 10 units respectively.
classifier = tf.contrib.learn.DNNClassifier(feature_columns=feature_columns,
                                              hidden_units=[10, 20, 10],
                                              n_classes=3,
                                              model_dir="/tmp/iris_model")

What does model_dir refer to?

After running the program, I am unable to find /tmp/iris_model.

This snippet is taken from: https://www.tensorflow.org/get_started/tflearn