2 Samuel 7:12 - And when thy [David] days be fulfilled, and thou shalt sleep with thy fathers, I will set up thy seed after thee, which shall proceed out of thy bowels, and I will establish His kingdom.
God was making the promise to David that a future generation of his direct lineage will become a King. This future King was Jesus. Refer to the gospels of Matthew and Luke for more information on Christ's bloodline.