"Neckbeard" is a pejorative online term to describe a person who exhibits social awkwardness, underachievement, and pretentiousness.
It's a reference to the poor grooming and hygiene standards associated with these folks, and their facial hair in particular, which, according to the stereotype, is unkempt and extends down their neck.
This doesn't mean that every guy with man fur under his chin automatically becomes one. Plus, people's personalities are so nuanced that it's often impossible to make an accurate assessment of them from something they say on the internet.
