in-place in here means that there is no extra memory allocation.
One more example, when you do
a = a + 1

you allocate a new tensor whose value is a + 1, and you assign it to a tensor called a, overwriting the previous reference to the tensor a. Still, the memory for a+1 had to be freshly allocated.
But when you do
a += 1

no extra memory is allocated, and the addition is performed directly in the original elements of a.