The prophet is commanded to prophesy a word of Blessing is the asking for or the giving of God's favor. Isaac was tricked into blessing Jacob instead of his firstborn Esau. At the Last Supper Jesus offered a blessing over bread and wine. To be blessed is to be favored by God. More to the mountains of Israel. They will yield much fruit.
God is no longer against the mountains but for them. They will multiply, and the people will inhabit the land again. It is a promise of a future for the land. (Compare with A prophet during the Babylonian exile who saw visions of God's throne-chariot, new life to dry bones, and a new Temple. More 6.)