Scientists long ago realized that atmospheric pressure drops as your altitude increases, as the atmosphere gets thinner the higher up you go.
Thus a barometer reading in Seattle would be a lot different than a reading in Denver at the same weather conditions.
To "level" the playing field, so to speak, scientists developed a way to calculate what the pressure is at sea-level, no matter what your altitude is. That way, you take altitude out of the equation and can see how weather is affecting the pressure where you are.
If you know your altitude, you can roughly calculate sea level pressure by subtracting .06 inches for every 50 feet you go up in altitude.
See more on baromters from Dec. 31's AskSteve column.