img = np.transpose(img, (2, 0, 1))

This code uses the numpy.transpose() function to rearrange the dimensions of a 3D numpy array called 'img'. The new order of the dimensions is specified by the tuple (2, 0, 1), which means that the third dimension (index 2) of the original array will become the first dimension of the new array, the first dimension (index 0) will become the second, and the second dimension (index 1) will become the third.

In other words, this code is transposing the image array from having dimensions (height, width, channels) to having dimensions (channels, height, width). This is a common operation in image processing when working with deep learning frameworks such as PyTorch or TensorFlow, as these frameworks typically expect image data to be in this format.