A portmanteau of "dork" and "adorable." It's an affectionate descriptor for someone who has traditionally dorky or awkward traits, but instead of being off-putting, they are seen as endearing. The traits in question can be physical or based on personality.
Dorky traits include things like clumsiness, nerdly enthusiasm, or just general social awkwardness. The traits alone, however, do not adorkable make (as I'm sure many of us here can attest to). The implication is that someone who is adorkable also has a sweet personality or sense of humor. Like most things, adorkable lies in the eyes of the beholder.