Thermodynamic instability is created by either increasing the rate of cooling with height or adding moisture to the low levels of the atmosphere. Either warm air advection in the low levels of the atmosphere or cold air advection in the middle and upper levels of the atmosphere OR a combination of both can increase the rate of cooling with height. Warm air advection is a quasi-horizontal wind which advects (or brings) warmer air into the forecast region. If low level warm air advection is accompanied by moisture advection, it is just that more unstable.

In the standard atmosphere, the temperature decreases with height. Why then doesn't precipitation develop almost every day since cold upper level air is just about always above surface warm air? It is not enough just to have cold air above warm air. There must be a trigger mechanism such as a front or low-level convergence / upper-level divergence that causes the air to rise OR the cold air must be much colder than the low level warm air (creating a steep enough lapse rate to produce convection and positive CAPE). When a forecaster mentions the atmosphere is becoming thermodynamically unstable, the forecaster is referring to a temperature profile of the atmosphere which is increasing in positive CAPE, is increasing in low level temperature and moisture, and/or a trigger mechanism (such as daytime heating, low-level convergence, upper level jet, positive vorticity advection) will in the near future cause low level air to rise into the upper levels producing deep convection in an environment with instability.