Have you ever visited a website and then ads from that website start popping up on Facebook and Google? Perhaps you were checking out some new shoes on a web store and added a pair to your cart but then aborted mission (because your partner says you have more shoes than Imelda Marcos). Later that day ads start coming up on Facebook, reminding you to complete the transaction. That's remarketing.
If you already have an AdWord campaign driving traffic to your website, a Facebook pixel is a must. Every time someone who is logged into Facebook visits your website, Facebook will track them and allow you to send out campaigns to those people. If they have been motivated to visit your website, they are more likely to become a future conversion.
Impressions on Facebook are fairly inexpensive, making it easy to build a relationship with your audience, even if they don't follow your Facebook page.