An image has Reflective Symmetry if there is at least one line which splits the image in half so that one side is the mirror image of the other.
The "Line of Symmetry" (shown here in red) is the imaginary line where you could fold the image and have both halves match exactly.
Draw a vertical line down the middle of an object, the two sides will be mirror images of each other.
Draw a horizontal line across the middle of an object, the two sides will be mirror images of each other.
Draw a diagonal line down the middle of an object, the two sides will be mirror images of each other.