Off Page Optimization is a method of Search Engine Optimization (SEO) in which you build links for your website so as to make it reach millions of people using internet. This would help you in getting rankings in search result.
Some of the techniques used are:
Directory Submissions: Submitting your website in web directories, just like you have your phone number in the telephone directory.
Social Bookmarking: Saving your links on social bookmarking sites like diigo, digg, folkd, delicious, jumptags, slashdot and many more. This is just like you bookmark any website in your Bookmark Menu.
Article Posting/Submission: This is the way by which you can sell your content, means tell the audience about your product/service.
Blog Creation/Posting/Submission: Same as Articles.
Forums Posting: These are discussion boards where you can write about your website and attract audience.
Apart from these 4 there are other techniques as well like Search Engine Submission, Business Listing etc.
The main reason behind doing Off page optimization is attracting traffic and visitors to your website.
Remember one thing: Target the right audience to get maximum result.