It can be a boolean or a string:

True or 'longest': pad to the longest sequence in the batch (no padding is applied if you only provide
    a single sequence).
'max_length': pad to a length specified by the max_length argument or the maximum length accepted
    by the model if no max_length is provided (max_length=None). Padding will still be applied if you only provide a single sequence.
False or 'do_not_pad': no padding is applied.