In a nutshell, the Software Product Line approach aims to achieve economies of scale in software production. This is how it's achieved:
" Each product is formed by taking applicable components from the base of common assets, tailoring them as necessary through preplanned variation mechanisms such as parameterization or inheritance, adding any new components that may be necessary, and assembling the collection according to the rules of a common, product-line-wide architecture. Building a new product (system) becomes more a matter of assembly or generation than one of creation; the predominant activity is integration rather than programming. For each software product line there is a predefined guide or plan that specifies the exact product-building approach."
In addition, industrialising software development isn't just about improving the technical or programming practice, it really is about many other important disciplines such as organisational, process, management, and architecture. Linda's seminal book on the subject presents the method, lessons learnt, best practice, and case studies (on how various organisations have successfully adopted software product line) to help you to get your organisation ready for the software industrial age!