The official documentation.
The atmospheric pressure that you would like PyEphem to use when computing how much objects are refracted at the horizon, which makes them appear higher (and thus rise earlier, and set later) than they really are. You can change the default value of 1010.0 millibar if you know that the weather where you will be observing will be different.
If you set the pressure to zero, then you will turn off the PyEphem refraction routine entirely. This will make calculations near the horizon slightly faster, and will return the altitude that objects would have if the Earth had no atmosphere and you could see straight into space.