Typically used proceeding a # when updating your social media status, nerdswag denotes that you believe you're activities are typical of the stereotypical nerd. A "#nerdswag" in a post necessarily implies that you don't care what people think of your nerdy activity or achievement.

An easier way to think of it, is simply looking at the word. Nerd + swag = nerdswag. All the intellectual strengths of being nerd, and all the social benefits of having swag.