Reflection
Reflections are everywhere ... in mirrors, glass, and here in a lake. ... what do you notice ? 

Every point is the same distance from the central line !
... and ...
The reflection has the same size as the original image
The central line is called the Mirror Line ...
Can A Mirror Line Be Vertical?Yes. 
In fact Mirror Lines can be in any direction. 

A reflection is a flip over a line
You can try reflecting some shapes about different mirror lines here:
How Do I Do It Myself?
Just approach it stepbystep. For each corner of the shape:  
1. Measure from the point to the mirror line (must hit the mirror line at a right angle)  2. Measure the same distance again on the other side and place a dot.  3. Then connect the new dots up!  
Labels
It is common to label each corner with letters, and to use a little dash (called a Prime) to mark each corner of the reflected image.
Here the original is ABC and the reflected image is A'B'C'
Some Tricks
XAxis
When the mirror line is the xaxiswe change each (x,y) into (x,−y)
YAxis
When the mirror line is the yaxis
we change each (x,y) into (−x,y)
Fold the Paper
And when all else fails, just fold the sheet of paper along the mirror line and then hold it up to the light !