3And I will grant authority to my two witnesses, and they will prophesy for 1,260 days, clothed in sackcloth.”
A witness is someone who has seen (or heard) a particular event (or saying) and subsequently passes on precisely what he’s seen (or heard). In this case the witnesses testify to what they have seen the Lord Jesus Christ do and/or say.
3 And I will grant authority to my two witnesses, and they will prophesy for 1,260 days, clothed in sackcloth.”