Lots of info about offset on the web, offset whether positive or negative, is the amount in mm the centerline of the tire is offset from the mounting flange face of the wheel hub itself. Most frontwheel drive cars have a positive offset while rear while drive cars have smaller positive offsets and some older cars even had zero offset where the centerline of the tire was inline with the wheel hub flange mounting surface. Be sure to get the right offset because wrong offset can affect handling adversely and/or wrong offset might cause tires to rub chassis/body parts when turning. Using spacers is not really a good idea either IMHO unless they are very precisely machined, and even so odd problems may occur with spacers if wheel nuts are not torqued correctly or if there is any rust, corrosion or dirt on the wheel hub flange surfaces.