By definition, "offset" is the distance from the mounting surface of the wheel to the "middle" (aka centerline) of the wheel.
E.G. You have a wheel that's 200 mm wide. The "middle" of the wheel (or centerline, "CL" in Magik's pic) would be 100 mm from the front and 100 mm from the rear.
If the mounting surface (where the wheel attaches to the rotor/drum) was in the "middle" of the wheel there would be 0 (zero) offset.
As the mounting surface is moved closer to the face of the wheel (aka the "outside") the offset becomes "Positive". As the mounting surface is moved toward the "inside" of the wheel the offset becomes "Negative".
Since the mounting surface moves one way, the entire wheel, when mounted, moves the other way. A wheel with 48 mm positive offset (the mounting surface is 48 mm from the centerline of the wheel, toward the face) will sit under the car more (aka "tucked in").
A wheel with 48 mm negative offset (the mounting surface is 48 mm from the centerline of the wheel, toward the back) will sit less under the car (aka "sticking out").
Therefore a wheel with a 42 mm positive offset (42 mm + offset) will "stick out" more than one with 48 mm positive (48 mm + offset).