During maturation, a portion of the volume of liquid in a cask is lost to evaporation, and is colloquially known as the Angel's Share.
The Angel's share is influenced by several factors. In hotter or dry climates the water will evaporate faster than the alcohol, increasing the ABV. In colder more humid climates the alcohol will evaporate faster lowering the ABV of the spirit.
The Angel's share is also responsible for reducing higher volatility chemicals in the spirit. These chemicals will evaporate through the cask faster than the lower volatility chemicals. This is equivilent to an extra "heads cut". This is important in making a pot still spirit better as it helps reduce the effects of smearing across the run. The effect isn't as profound with spirit made with higher rectification stills like multi-plate columns or other HETP stills.