If you were told that two identical twins are congruent and asked to explain why they are called congruent, what would you say? What would distinguish identical twins from a random third person? Obviously, the twins are similar because they look alike, share the same body type, and may even have the same measurements in terms of height, weight, etc. Likewise, we can think of congruent triangles in the same way, because their corresponding angles and side lengths will be identical.
Learn this and go on to Notation for Congruent Parts