Essentially, the lapse rate is a measure of how much air decreases in temperature as it rises through the atmosphere. Generally this is about 6.5 C per 1000 m. This rate does vary and depends on local air conditions.
Lapse rates are lower in winter or during a rainy season.
Lapse rates are lower over land than sea. Different properties of air masses mean different lapse rates. A dry parcel of air which does not mix at all with the surrounding air is considered. As the parcel of air rises through the atmosphere the surrounding pressure is less and so the parcel expands. This then means that the saturated lapse rate is used below this temperature.