What does the '[103.939, 116.779, 123.68]' means?

[vba34520]

• Which backend do you use (TensorFlow or Theano) and what its version?
  TensorFlow
• What version of Python?
  3.6.4
• What version of Keras?
  2.2.4

Thank you for your working firstly.
I have a question about the xxx.mean = np.array([103.939, 116.779, 123.68], dtype=np.float32).reshape((3, 1, 1))

What does the '[103.939, 116.779, 123.68]' means?
Does it means the datagen should reduce the imagenet's BGR mean value?

Looking forward to your reply. Thank you very much!