To check if two line segments are congruent,you just need to check their
lengths and if lengths are equal then they are congruent.
To check if two angles are congruent,you just need to check their measures and if the measures are equal then they are congruent.
Extending this concept to triangles,If two triangles are congruent,then they must be copies of each other and when superposed they cover exactly each other.
To review your concepts, you can refer to Review Notes. To test the concepts you just learnt,try the questions in Question set 1.