The first A refers to the bottom left pairs of angles for the triangles. They have one single arc mark to indicate they pair up and are congruent.
The S refers to the bottom sides of the triangle having the same length (the single tickmark shows they are congruent)
The second A is for the pair of angles on the bottom right side of the triangles. These angles have 2 arc marks.
The order of the lettering is important because the S in ASA is between the two angles. As the diagram shows, the sides mentioned are between the angles mentioned. Contrast this with AAS or SAA which has the sides not between the angles.
