1- In its oldest form, sackcloth represented humiliation. People were dressed in sackcloth to signify- that they were less than dust. Hence the saying ‘sackcloth and ashes’. Nowadays in dreams it is much more likely to represent our own sense of humiliation, that we have humiliated ourselves in an action which we have taken.
2- Sackcloth also indicates repentance. We may feel that we have humiliated ourselves, but equally we may wish to show the world that we have repented of an action or deed.
The symbol of sackcloth comes up in a dream to indicate that we need to make an outward show of such repentance.
3- The act of mourning in olden times often indicated some public show, and so sackcloth was taken as the substance to show the spiritual poverty of the people concerned that thev had lost some- thing very valuable. In dreams this symbolism can also come across.... Ten Thousand Dream Dictionary