This matrix allows us to perform affine transformations in our scene. Affine is a mathematical name to describe transformations that do not change the structure of the object that undergoes such transformations. In our 3D world scene, such transformations are rotation, scaling, reflection shearing, and translation. Luckily for us, we do not need to understand how to represent such transformations with matrices. We just have to use one of the many JavaScript matrix libraries that are available online (such as glMatrix).
Note
You can find more information on how transformation matrices work in any linear algebra book. Look for affine transforms in computer graphics.
Understanding the structure of the Model-View matrix is of no value if you just want to apply transformations to the scene or to objects in the scene. For that effect, you just use a library such as glMatrix to do the transformations on your behalf. However, the structure of this matrix could be invaluable information...