The dotted line down the middle of the letter A, below, is called a mirror line, because if you place a mirror along it, the reflection looks exactly the same as the original.
Another name for a mirror line is a line of symmetry. This kind of symmetry can also be called reflective symmetry or reflection symmetry.
Now look what happens when you fold the letter 'A' along different examples of dotted lines.
Only in the third example does the folded part of the shape fit exactly over the original part, so this is the only example showing a line of symmetry.