Scientists called meteorologists use a special instrument called a barometer to measure the pressure of the atmosphere. What exactly is it that they are measuring? What is pressure?
To understand pressure, consider a sponge. When left alone, a sponge stretches out to its full size. As you squeeze down on the sponge, what happens? How small can you make it? Can you hide the sponge entirely within your hand? When the sponge is squished up as tightly as you can make it, how does it feel? The sponge should feel hard and tight. Now, as you let go of the sponge and it once again takes its original size and shape, how does it feel? In this state, it should feel soft again.
The atmosphere is similar to a sponge in that it can be compressed, or squished up. The weight of the air above compresses the air below tighter and tighter. The lower we travel down into the atmosphere, the more weight there is above, and so the tighter the air is squished or compacted. The higher we travel into the atmosphere, the less air there is above and so the less weight there is pushing down, and as a result, the less squished up, or the less compact the air.
Half of the air making up our atmosphere is squished down into the first three layers, and 90% of the atmosphere is squished into the first 10 miles (16km). Above this altitude the air is so thin that the pressure is even less than that of the best vacuums on the surface of the Earth.