You will find a lot of answers here: http://www.google.com/search?hl=en&q=what+is+link+development?&btnG=Google+Search&meta=
Link development is obtaining links from other sites back to yours, as well as linking out to other sites, although that is a relatively minor element of developing links unless you are involved in reciprocal. While a good question, a great one even it is not something one can answer without some background. Since links into your site can/are viewed as a vote for your site then developing links=votes to your site from quality sites is essential for search engine ranking and traffic to some degree. Hope this helps some!