What does -1 mean in numpy reshape?

A 2D array can be reshaped into a 1D array using .reshape(-1). For example:

>>> a = numpy.array([[1, 2, 3, 4], [5, 6, 7, 8]])
>>> a.reshape(-1)
array([[1, 2, 3, 4, 5, 6, 7, 8]])

Usually, array[-1] means the last element. But what does -1 mean here?