Two triangles are similar if -
1Their corresponding angles are equal in measure.
2 Their corresponding sides are proportional
now,AA simillarity means Angle angle similarity.
If Two triangles   have two corresponding angles that are congruent or equal in measure.then we say they are similar by AA similarity criteria.

If two angles of one triangle are respectively equal to two angles of another triangle, then the two triangles are similar.
proof : 

Let ΔABC and ΔDEF be two triangles such that ∠A = ∠D and ∠B = ∠E.

∠A + ∠B + ∠C = 180
 0 (Sum of all angles in a Δ is 180)

∠D + ∠E + ∠F = 180
 0 (Sum of all angles in a Δ is 180)

⇒ ∠A + ∠B + ∠C = ∠D + ∠E + ∠F

⇒ ∠D + ∠E + ∠C = ∠D + ∠E + ∠F (since ∠A = ∠D and ∠B = ∠E)

⇒ ∠C = ∠F 

Thus the two triangles are equiangular and hence they are similar by AA.

I want proof of this theorem
can you proove