Answer:
If two triangles are congruent, then by the definition of congruent triangles, 3 pairs of sides are congruent and 3 pairs of angles are congruent. In all, this is 6 statements of congruence.
There are ways of proving triangles congruent that require only 3 statements of congruence of corresponding parts, such as ASA, SAS, AAS, etc.
Sometimes in order to prove two segments or two angles congruent, since these segment and angles are parts of triangles, you prove the triangles congruent using some other statements of congruence, not involving these parts, and then you get 6 statements of congruence that includes the parts you wanted to prove congruent. That is where you use "corresponding parts of congruent triangles are congruent", or CPCTC for short.