To be congruent two triangles must be the same shape and size. However one triangle can be reflected, that is, flipped over with respect to the other. As long as they are otherwise identical, the triangles are still congruent. In the figure below, the triangle LMN is a reflection (mirror image) to PQR, but the triangles are still congruent.
One way to think about triangle congruence is to imagine they are made of cardboard. They are congruent if you can slide them around, rotate them, and flip them over in various ways so they make a pile where they exactly fit over each other.
In the figure above, if the triangles were cardboard, you could take LMN, turn over and it would fit exactly over PQR.