meaning of syntax : ”encoded_box[:, -1][assign_mask] = iou[assign_mask]”

[AllanZuo]

I don't anderstand the following codes:
what is in encoded_box after execute this code？
encoded_box[:, -1][assign_mask] = iou[assign_mask]

And what is the output of ”encoded_box[:, :2][assign_mask] ”？

encode_box is a tensor ande shape likes [[xmin,ymin,xmax,ymax],...].assign_mask is a tensor of [true,false,.....].
i just can't anderstand the syntax.who can help to explain it?