When an image is observed by placing a mirror beside it, we can see the reflection of the image on the mirror, and we observe that the reflected image is the same as the original, this is called reflection symmetry.
Figure-1: The black surface is acting as a mirror, and we observe the reflection of shapes on the mirror, which are symmetric along the mirror surface. This mirror surface is called the mirror line along which the shapes are reflecting and showing symmetry.
Figure - 1
We can see reflection symmetry in nature along the side of water bodies (Figure - 2), here we observe the reflection of the building and trees caused on the surface of the water.
Figure - 2
Reflection symmetry is a phenomenon that we see around us in our day-to-day life. It's important to study and understand it.
In simple terms, we can say symmetry caused by one (or) half of an object is a mirror image of the other.
Line symmetry and reflection symmetry are naturally interlinked to each other. A shape has line symmetry when one half of it is the mirror image of the other half. A mirror line thus helps to visualise a line of symmetry (Example, figure-3 the line doesn't show symmetry; whereas figure-4 has symmetry). When a paper is folded in half, the mirror line acts as the line of symmetry.
Not symm reflection.svg
Figure - 3
Figure - 4

When can we say the image is the reflection of the object along the mirror line?
When an object is reflected, there is no change in the length and angle of the reflection (physical appearance); i.e. the length and angle of the object and the corresponding length and angle of the reflection are the same. With one exception; there is a difference between the object and reflection when left and the right sides of an object appear opposite (inverted) in a mirror. An example is shown in figure - 5 alphabet '\(S\)'.
The object and its reflection are symmetrical along the mirror line.
Figure - 5