I will explain this but it is another one where you are probably better off leaving it to us if you want this level of work performing upon your site.
The internet is all about interconnectedness. Within the context of your website you need a navigation structure to ensure that the visitor can find all of the information that they require in as few steps as possible. Stretch that context to encompass the world wide web and you need to secure yourself a position within this complicated information flow by linking to the relevent sites whose visitors may be interested in your products and also getting them to link to you.
The search engines view such interconnectedness as desireable and reward it with higher search engine rankings. They also penalize your search engine rankings if you link to irrelevent sites, or too many sites, or the wrong type of site.
The payback for a corect linking strategy is a higher hit rate on your site and if your conversion rate from visitors to sales is good then more sales will follow. This will only happen however if you get it right.
At first glance this seems simple. If you think that it is then go for it. Who knows, you may get it right. If you don't. Remember where we are and come back.