Hi,
It does the backward pass wrt the input. So the reverse of the forward.
Given the names, it performs unpooling (I guess the forward has some pooling builtin.
Then im2col that creates the Teoplitz matrix of the input (https://en.wikipedia.org/wiki/Toeplitz_matrix 5).
Then a matrix matrix multiplication between this and the weights.
This will give the result. (the backward of a conv is a conv. Here implenented using a Toeplitz matrix).