When your friend squeezes your arm, you feel pressure! That's because molecules collide with each other and things like your arm, the ground, or a tree. They exert a force on those surfaces. Air has pressure too.
Air molecules constantly bounce off each other and everything else around them. The force exerted by these air molecules is called air pressure. Where air molecules are packed closely, air pressure is high. Where air molecules spread out, air pressure is low. For example, the air molecules inside this balloon are at higher pressure than the molecules outside the balloon.
Air Pressure and Weather:
What's a high pressure system?
Sometimes, high in the sky, air slows its forward motion and piles up. This is called convergence. In the piled-up air, molecules are packed closer togehter. This makes the air more dense, so it sinks. When the sinking air reaches the Earth's surface, it spreads out. This is called divergence. In the Northern Hemisphere, it spreads out while rotating clockwise near the ground. This is called a high pressure system.
What's a low pressure system?
Sometimes, high in the sky, air moves away from an area. When air spreads out (divergence) at high altitudes, air from below flows upward to fill the space, making a low pressure system. Air converges when it gets to the ground and turns counter clockwise in the Northern Hemisphere.