The air is essentially a mixture a various particles of molecules in constant random motion, and constantly colliding with each other. The outer space on the other hand, has no molecules, hence it is a vacuum.
Pressure exists due to all of these particles and molecules around us, constantly colliding with each other, and onto the surface of the wall, and even your body. The summation of these collision is pressure. That is what the formulae to calculate pressure is Pressure = force/area. in other words, "total summation of force generated from molecules collision unto a specified piece of area"
a way to look at it, more molecules, results to more collision between particles and wall. So the more air you blow into a balloon, the more air particles there are in the balloon, the more force it has against the walls of the balloon, the balloon will then expand as there is an increase in pressure as you keep on blowing