Thin layer of the atmosphere where the normal decrease in temperature with height switches to the temperature increasing with height, and as a result, warmer air ends up on top of a colder layer. Temperature inversions are very stable profiles, which stop convection in this layer. This means that there is almost no mixing of the air. Under these circumstances, emissions do not rise to higher altitudes, pollutants from the surface can become trapped close to the ground.
This regular temperature inversion in the high Arctic is caused by the lack of surface heating by the Sun, which stays below the horizon during the winter (and polar night), and the continuous loss of heat from the surface (through emission of infrared radiation).
Because of stratified conditions, the efficiency of that heat transport decreases. That means that what little heat is emitted by the cold Arctic surface can accumulate in the lower atmosphere, raising temperatures there and contributing to Arctic amplification.