It’s the number of batches used for training in each epoch. In this case the model is trained on all available train samples every epoch, which total 1875 batches. Then it is evaluated on all test samples, which total 313 batches.
If you divide the number of samples by the batch size, you should get the same numbers .
In this basic example the number of batches just shows the training progress. But there are some use cases, when it could be beneficial to set the “steps_per_epoch” argument in fit() to some smaller number of batches and validate the model more often.