Just one in general

An article (a, an, some, the) gives information about the noun in the noun phrase. It can:

• Tell us how many there are; if the article is “a” or “an,” we know there’s only one.
• Tell us whether the noun in question is a specific one or just one in general.
• Signal to a reader or listener that a noun is just being introduced or that it’s one he’s already seen in a story.

Read more at http://grammar.yourdictionary.com/grammar-rules-and-tips/definite-and-indefinite-articles.html#z6VIzAh42aOSUKx3.99

Could you please elaborate on the "one in general" part or type?