In numpy tutorial [1, 2, 1] is an array of rank 1, because it has one axis. That axis has a length of 3.
[[ 1., 0., 0.],
[ 0., 1., 2.]]
The above array has rank 2 (it is 2-dimensional). The first dimension (axis) has a length of 2, the second dimension has a length of 3.
Can someone explain these terms, if possible with examples?