Realistic Atmospheres modifies the atmospheres of all planets and moons to conform to a more lifelike model. Temperature-height profiles are based on real life celestial bodies of similar type. Pressure-height models have been developed consistent with each celestial body's physical characteristics, and using real life gas laws. Models have been developed at real scale, and then scaled down to better fit the smaller size of the KSP universe.
While some properties of the original atmospheres have been preserved, Realistic Atmosphere is a complete makeover. Don’t expect what you know about the stock game to exist in Realistic Atmospheres. You may encounter a planet's atmosphere much sooner or later than you are accustomed to, and with different behavior, so beware.
Installation instructions are in the README included in the download.