Hearken is an old fashioned form of the word hark, meaning "to listen" (see hark). In the Bible, prophets and saints are always telling people to hearken to their words.
While hark is still used today, hearken is pretty much obsolete — unless perhaps you happen to be an old school preacher. Hearken, like hark, can also mean to look back to something in the past, though there is often a critical sense to the word. To hearken back to an old memory or event is usually to do it in a way that may be annoying to other people or perhaps even damaging or self-defeating to the one doing the hearkening. Frequently spelled harken in the US.