Congruence theorem here means one out of four theorems commonly used in conjunction with triangles. The theorems tell you what you need to know about a triangle in order to draw it with all its sides, angles and areas in an unambiguous way. Here are the four theorems commonly used in German maths at school and university:
SSS is German for SSS in English: three sides are given.
SWS is German for ASA in English: two sides and the angle between them are given.
WSW is German for ASA in English: two angles and the side between them are given.
SsW is German for SsA in English: two sides and the angle opposite the longer side are given.