By default, when you render a scene, you are looking at it from the origin of the world in the negative direction of the z-axis. As shown in the following diagram, the z-axis is coming out of the screen (which means that you're looking at the negative z-axis):
From the center of the screen to the right, you will have the positive x-axis, and from the center of the screen up, you will have the positive y-axis. This is the initial configuration and it is the reference for affine transformations.
In this configuration, the Model-View matrix is the Identity matrix of rank four.
The first three rows of the Model-View matrix contain information about rotations and translations that are affecting the world.