To build is to construct or make something. When you build a house, you start at the very bottom, with the foundation, and work your way up.
You build a birdhouse using scraps of wood, a hammer, and nails, or you can build a career as a poet by writing daily and submitting poems to literary magazines. In both examples, you're constructing something yourself using materials and hard work. Another meaning is to make stronger: "I have to build my endurance if I'm going to finish that marathon." The Old English root of build, byldan, specifically means "build a house," from bold, "house."