Ezekiel was one of the Israelites taken captive at the fall of Jerusalem and moved from his home in Jerusalem to Babylon. Basically his world has been destroyed - his home ransacked - his career as a priest crumbled when the temple was demolished and he was taken off along with thousands of others.
So instead of being a priest leading the people in temple worship, he becomes a prophet - seeing visions from the Lord and hearing the words of the Lord. In this visitation of the Lord, Ezekiel is transported to a valley and the valley is filled with bones - dry bones - that seem to have been there for centuries. And the Lord asks him, "Can these bones live?" Well, what would you say? Ezekiel, says, "You know" - implied in that is "how should I know? You're Lord, you tell me." And God tells him to prophesy - prophesy to the bones.