What it does is row-wise merging. This post has some nice example:
>>>V = array([1,2,3,4,5,6 ])
>>>Y = array([7,8,9,10,11,12])
>>>np.r_[V[0:2],Y[0],V[3],Y[1:3],V[4:],Y[4:]]
array([ 1, 2, 7, 4, 8, 9, 5, 6, 11, 12])
Read more about it in this , and in the documentation of numpy.