A backlink anchor is the anchor text associated with a backlink.
Backlink anchor optimization is important for SEO but delicate. Indeed backlink are more often created independantly by third parties without the website owner being aware of it.
Some practices allow to control backlink anchors redaction, but those practises are often black hat SEO.
Since its algorithm update in spring 2012, called Penguin, Google seems to search to penalize or to reduce the recognition of over-optimized backlink anchors. Thus, the presence of a great number of identical anchors for the same website or page can now carry a risk of penalty.
For more details and explanations about backlink anchors, see anchor text.