Yahweh tells Israel that he does not change (v. 6) and therefore stands ready to renew covenant blessings as soon as Israel returns to its covenant obligations. He repeats that message here.
The reference to “meat in my house” means animal sacrifices in the temple which also provided the priests’ food. When tithes and offerings come to the temple in the proper order, then the priests can observe proper temple ordinances.
In the Hebrew conception of the universe, the earth was covered by a heavenly tent that kept out the celestial waters. When it rained, the “windows of heaven” or the windows of this tent opened, and celestial water came down to earth. Malachi transforms this image from water to blessings. The rain falls over large areas and creates beneficial conditions on the entire earth. These symbolic windows are holding back blessings; opening them will, like the rain, benefit a large area. The image is of a barrier being removed. Like the heavenly waters that are prevented from falling, opening the windows of heavens will permit the waiting blessings to fall unchecked on Yahweh’s children. The point is Yahweh’s willingness to keep his part of the covenant relationship.