な is used by na-adjectives to modify the word that follows them. This is exactly the same way の is used with nouns to modify the word that follows them. な derived from なる - and なる can still be used in some circumstances in the same way.
E.g. 車 - (a) Car. Modify with な-adjective, 大きな車 - (a) Large car. Modify with noun フレッドの車 - Fred's car.
Note that the same word can often be used both as a na-adjective and as a noun. Also quite a lot of i-adjectives can (with the i removed) be used as na-adjectives. e.g. 大きい車 vs. 大きな車