In verse 6 the Lord tells Israel that he does not change, and therefore stands ready to renew the blessings of the covenant as soon as Israel returns to its covenant obligations. This is the theme here as well. If Israel returns to its obligation to bring the tithes and offerings to the temple, then the Lord is ready to pour out blessings.
[that there may be meat in my house]: This reference is to the meat for sacrifices in the temple. Thus the tithes and offerings come to the temple, and proper temple observances may be kept.
[windows of heaven]: In the Hebrew conception of the universe, the world was seen as being covered by a heavenly tent that kept out the celestial waters. When it rained, the windows of the tent were opened and some of that celestial water came down to earth. This imagery is transformed from water to blessings. The rain falls over large areas and creates beneficial conditions on the entire earth. The opening of these symbolic windows that are holding back the blessings will similarly benefit many over a large area. Just as with the celestial waters, the image is one of a barrier that is being removed. This emphasizes the role of the Lord in his willingness to keep his part of the covenant relationship.