Lets make this easy on all of us. The offset of a wheel is the distance from its hub mounting surface to the center-line of the wheel. The whole point of an offset is for the end user of the wheel to know how much in inches (4+3) or in the case of millimeter offset, how many mm the wheel will stick out or suck in from the mounting surface of the hub. Remember, even a couple of millimeters can affect performance, ride quality, the look/stance of your vehicle and more important than anything, how safe the setup is.
Negative Offset means your wheel will stick out further because your wheel hub will now be closer to the inside of the wheel (left picture in diagram below).
Zero Offset means your wheel hub is now centered on the wheel (middle picture in diagram below).
Positive Offset means your wheel will be "sucked in" this is because your wheel hub is now closer to the outside of the wheel (right picture in diagram below)