A surface in the air where pressure is equal is called a constant pressure surface, or isobaric.
Since pressure decreases as you get higher, an altitude with a low pressure is higher than one with a high pressure (100mb is higher than 500mb).
Pressure decreases, because there is less air above for example 50km than 5km. Most of the air is close to the ground, because of gravity. This is why air pressure decreases slowly at first, and rapidly when you get higher.
When the pressure is half of the air pressure at the ground (about 1000mb, thus 500mb), it means that half of the air is underneath it, and the other half above. In the image at the right you can see that the 500mb line is underneath the half of the atmosphere. This image is just an example. The pressures can change every day everywhere.

Pressure	Approximate height	Approximate temperature
Sea level	0m	15C (59F)
1000mb	100m (300ft)	15C (59F)
850mb	1500m (5000ft)	05C (41F)
700mb	3000m (10000ft)	-05C (23F)
500mb	5000m (18000ft)	-20C (-04F)
300mb	9000m (30000ft)	-45C (-49F)
200mb	12000m (40000ft)	-55C (-67F)
100mb	16000m (53000ft)	-56C (-69F)

Constant pressure surfaces and height
The constant pressures are important for weather balloons, since they measure height by the air pressure. On the left you can see some constant pressure surfaces. These are approximate numbers, since air pressure changes.

Data from the University of Illinois WW2010 Project.

Constant pressure surfaces and temperature
The height of a pressure surface can differ because of temperature. When temperature is equal, the height of a constant pressure area is also equal. But when one constant pressure area is cooler than the other, the height differs.
This happens because cooling air contracts, thus decreasing the height of a pressure area. When air warms up, it expands and increases the height of a constant pressure area.

Coriolis is the force which develops because of the rotation of the earth. It makes global winds bow. See also: Global wind Global pressure movement