The simple definition is that it’s an image which is like a reflection in a mirror. Everything is the same, except reversed.
In symmetrical shapes, you can draw an imaginary line down the middle of the shape, and the objects on each side of the line will be mirror images of each other.
When the shapes are asymmetrical, the two shapes are unique and cannot be used interchangeably in a pattern. (i.e. one side will work, but one will not.) An example of where you need to be careful about mirror images, would be when cutting Half Rectangle Triangles as in the image above