The shape of the images training set is (60000, 28, 28). To clarify, there are 60,000 images in the training set. Also, the dimension of each image is 28 x 28 pixels.
The length of the labels training set is 60,000 due to there being 60,000 labels in the training set.
The shape of the images test set is (10000, 28, 28). This information means that there are 10,000 images in the test set. In addition, the dimension of each image is 28 x 28 pixels.