When a rocket launches into orbit, it needs to go very fast and to do that, it has to overcome two major obstacles. The first is gravity and the second is the air. Gravity doesn't change much. As the rocket gets higher off the ground and burns more of its fuel, it loses a lot of its weight and so the force of gravity gets weaker and weaker as the rocket goes higher and faster. But air doesn't behave the same way. As your rocket begins to really pick up speed, the air fights back harder and harder. At best, the air will sooner or later will push back on your rocket just as hard as your thrust is pushing you forwards and your rocket wont go any faster. At worst this huge air force will rip your rocket into pieces. SO to get around this problem, we launch our rockets directly up to get out of the thick, lower atmosphere as soon as possible. This does nothing more than reduce the distance your rocket has to travel through that nasty air. In the thin upper atmosphere, there's less air to slow you down. That's when you can reach the speed of orbit.