Some believe that reciprocal linking (your site links to my site, mine to yours) is worthless in the eyes of the search engines as they’re clearly not genuine ‘votes’.
To get round this, people engage in 3-way linking: say I have 2 sites hosted on different servers. My site A links to your site, your site links to my site B. We both get a link but the search engines are fooled into thinking they’re both genuine ‘organic’ links, not reciprocal, so they carry weight.
Hope this explains it a little!