Barometric Pressure affects firearm ballistics. High pressure tends to increase air density. With air that is less dense the bullet as it travels encounters less resistance and is able to conserve its velocity longer.
If you do not check the 'Correct for Atmosphere' checkbox then the International Standard Atmosphere pressure of 29.92 hg will be used.
The Weather Underground is a good website to use to find out the altitude, pressure, temperature, and humidity for where you live. Plug those numbers into the ballistic trajectory calculator in order to have more accurate results.