According to Mathwords, reflection is a transformation that creates a mirror image. This transformation occurs when a geometric figure is reflected across a certain line called the axis of reflection.
Reflection can also be called a flip. In a reflection, one object is transformed into a mirror image of itself; this is one of the three main methods of isometry, which is the act of moving something without changing its size or shape (within a single plane).
In isometry, you can rotate an object, which just means turning it; you can translate it, which is just a translation of the object's location (moving it without turning); and you can reflect it, which is just like looking at an object in the mirror. This flips the object over, creating a reverse image of the object the exact distance from the axis of reflection as the original object. This object is mirrored, meaning that the left and right sides of the reflection have changed places.
Reflection is the process that causes the most change; an object cannot achieve the same effect as reflection by translation or rotation nearly as easily.
The technical terms translation, reflection, and rotation can be substantiated for slide, flip, and turn, respectively. These simple words may help in visualizing and memorizing the processes involved.