The air in our world sits at a rather constant pressure, called the atmospheric pressure , of 760 mm Hg. If we want to draw air into our bodies, we have to be able to make our respiratory tract have a lower pressure than 760 mm Hg. It has to be lower, because all items tend to move from high pressure toward low pressure without any extra energy needed. Keep in mind that we have to do this over and over, since breathing is cyclical. To understand this, you may need to picture what air pressure really is.
Air is made up of many molecules.
All molecules are in constant, random motion. As the molecules move around, the amount that they bang against each other and against any container that they are in is their pressure. I have tried to demonstrate this for you in the animation below. At first, the air molecules are just moving around normally. Then, I have gone ahead and changed the container within which they are moving.