What Is Congruence, in Geometry?

If shapes are congruent, it means they're identical

If two shapes are congruent, it means that they share the exact same size and shape. In other words, all their sides are the same lengths and all their angles share the same measure (see the definition of angle congruence here). Location and rotation don't matter at all for congruence.

All the blue shapes in the image below are congruent to one another. The red shape is similar, but not congruent, to the blue ones.

Notice that all the blue shapes have congruent angles and sides. The red shape has congruent angles to the blue ones, but not equal sides, so it's not congruent.