"Clothed in sackcloth." II Kings 1:8 is the response of some people who reported what they had seen to the king, Ahaziah: "So they answered him, 'A hairy man wearing a leather belt around his waist.' And he said, 'It is Elijah the Tishbite." Matthew 3:4 describes John the Baptist: "Now John himself was clothed in camel's hair, with a leather belt around his waist; and his food was locust and wild honey." So Elijah and John the Baptist both wore sackcloth. In a way, they are types of these Two Witnesses.
Being clothed in sackcloth has several meanings in the Bible. They are all somewhat similar, but they have nuances that we need to consider.
Sackcloth was worn by those who were in mourning. Recall in Ezekiel 9 that the angel was supposed to mark all those who sighed and cried for all the troubles of Jerusalem. That is a sign of woe, of mourning, or of being sorry for the fall of this once great nation or for their sins.
Sackcloth also can mean repentance, as an outward sign of the inner repentance of a person. Therefore it also has another meaning of being humble. A repentant person should be a humble person. He has seen his sins and turned from them.
Another meaning is austerity. This is one that the world often sees in John the Baptist and Elijah, that they were "poor" men. However, that is not necessarily the case. Austerity does not necessarily mean that one is poor. It can mean though that a person leads a simple lifestyle, and that he has removed the frills that complicate his life. Wearing sackcloth, then, could mean a person has stripped down to the simplest essentials of his physical life.
Of course, the one that goes with this would then be poverty, yet not necessarily physical poverty (a lack of money) but spiritual poverty (poor in spirit). This is a fine way of looking at the wearing of sackcloth in the case of the Two Witnesses—and frankly, of Elijah and John the Baptist. They were ready to be filled and given the riches of God because they had considered themselves lowly and needy. They knew they needed what only God could give. They were poor in spirit.
However, all of these meanings could apply to the Two Witnesses: They mourn for the troubles this world is going through; they are repentant and humble; they are austere, not having any of the frills and complications that clutter other people's lives—they have stripped themselves of the things that would weigh them down so that they can run (Hebrews 12:1); and they are certainly poor in spirit.