Aspiration is a "breath" that follows a speech sound. Compare the sounds associated with the English letter t for example. Hold the back of your hand close to your mouth and say "Top." Then say "Stop." Can you feel the burst of air that follows the t in "Top" but not in "Stop"? The T in "Top" is aspirated. The one in "Stop" is not.
Aspiration is a significant part of the phonetic system of some languages. In Classical Greek pronunciation, for example, some letters must never be aspirated, while others must always be. Aspirating a sound that should not be aspirated would cause it to be understood as a different sound. Contrast the sounds of the letters in the table below.
|θ (Theta)||τ (Tau)|
|φ (Phi)||π (Pi)|
|χ (Chi)||κ (Kappa)|