Yahweh always causing one of David's descendants to be king is spoken of as if David's descendants are a building that Yahweh will build and make firm. (See: [[rc://en/ta/man/jit/figs-metaphor]])
Here "throne" represents the power to rule as king. The psalmist speaks of God promising that one of David's descendants will always rule as king as if God will build David's throne and make it firm. Alternate translation: "I will make sure that one of your descendants will rule as king over every generation of my people" (See: [[rc://en/ta/man/jit/figs-metonymy]] and [[rc://en/ta/man/jit/figs-metaphor]])