The atmosphere is in continual motion with changing temperatures, air currents or wind and weather fronts, causing stars to twinkle. (The images of twinkling stars are highly distorted and the amount of light reaching us through the air is also affected, so it changes from instant to instant and the star twinkles.) If the stars are twinkling considerably, this is known as "poor" seeing conditions. When the star images are steady, this is known as "good" seeing conditions. Poor seeing is most noticeable when observing planets, close double stars and the moon, whereas deep sky objects such as nebulae and galaxies are less affected by poor seeing conditions.
For deep-sky objects, the most important factor is the transparency of the atmosphere (a measure of how clear the sky is on a given night - determined by clouds, dust and haze). Seeing conditions and transparency will vary widely from site to site, from season to season and from night to night.