The noun prophecy is a prediction—a statement of what will happen in the future, traditionally from a spiritual, or mystical source, such as the Bible, or a holy person, such as a priestess, monk, or prophet. Prophecies are understood to come from a supernatural source such as a deity, rather than science or reason.
The verb prophesy has the same meaning, but it is followed by a noun or verb describing the future event:
Gina prophesied disaster.
Gerard prophesied that he would lose the battle.
In summary, prophecy and prophesy are a noun/verb pair with essentially the same meaning, but different pronunciations; the ending of the noun prophecy is pronounced like SEE, and the ending of the verb prophesy sounds like SIGH.