This prophecy speaks of two men of unknown origin who will be mighty prophets before the Jewish people during the Tribulation and will preach powerfully and be invincible, will kill their enemies with fire, and will have a ministry of 3 1/2 years after which they're killed, lay dead for 3 1/2 days, and then are resurrected and carried into Heaven. Their ministry is supposed to be during the first half of the 7 year Tribulation. Once they die it won't be long before the Great Tribulation, the second half of the overall Tribulation, will start.
There are some who believe that their work will precede the start of the Tribulation by a brief period as their ministry must be completed, as well as their death and resurrection, before the second half of the Tribulation begins. How long before the signing of the 7 year peace treaty and the start of the Tribulation is unknown. But likely it'll be no more than a week or two as that would still afford plenty of time for the events that must follow their carrying away into Heaven (the death and resurrection of the Anti-Christ and his desecration of the temple) to occur and come to completion before the Great Tribulation, the second half of the 7 years, begins.
These two witnesses, during the time of their ministry, will cause all rain to cease in Israel, and will repeat all of the plagues that Moses and Elijah called down upon the enemies of God, including fire, water turned to blood (as spoken of earlier), boils, flies, frogs, fire, famine, etc, and will do so as regularly as is needed in their ministry as they seek to turn Israel from her sins to her real Messiah, Jesus Christ. There are many theories on who these two men will be. Some candidates include Moses, Enoch, Elijah and several others. But since the bible doesn't name who they will be, it's most likely that the Two Witnesses will be entirely new people and will merely walk in the spirit and the mantle of the prophets of old in much the same way John the Baptist walked in the mantle of Elijah.
(*Do you have a better description, or did you find an error? Please let me know.