AAS, or Angle-Angle-Side, is a criterion used to prove the congruence of triangles. It states that if in two triangles, two angles and the non-included side of one triangle are congruent to the corresponding two angles and non-included side of another triangle, then the triangles are congruent. This method leverages the fact that when two angles are known, the third angle is automatically determined due to the properties of triangles.