Cacao nibs are essentially unprocessed chocolate. They are pieces of the cacao beans that have been roasted and hulled, but not yet ground into chocolate. The taste of these nibs bears some resemblance to coffee beans, although cacao nibs tend to be a bit crunchier.
Cacao nibs provide all the heath benefits of chocolate without any of the detrimental consequences of the milk and sugar that are normally added to cacao to make it tastier. These little nibs are full of antioxidants, particularly flavonoids which may impart benefits to cardiovascular health.
Mark Sisson has a whole list of reasons why we should be eating dark chocolate, especially the raw, unprocessed stuff, if we can get it.