Learn German: Der Rock passt nicht. - The skirt does not fit.

Yes! In everyday spoken German, the basic "the" words (der, die, das) are frequently used to mean "this" or "that". If you emphasize the word der when speaking, Der Rock means "That skirt". To be more formal and say "This skirt", you would use Dieser Rock.

Why is a skirt (Rock) a masculine noun using der?

Grammatical gender in German rarely matches up with biological gender or who typically uses an object. The word Rock simply belongs to the masculine category of nouns. It is always a good habit to memorize the article (der, die, or das) along with every new noun you learn.

Why does the word nicht go at the very end of this sentence?

When you want to negate the main action of a simple sentence (in this case, the fact that the skirt fits), nicht is placed at the end of the clause. It is somewhat like saying "The skirt fits... not."

Does the verb passen only apply to clothing?

While it is the exact verb you need when talking about clothes being the right size, it is also very common when making plans. If someone suggests meeting for coffee at 3 o'clock, you could say Das passt! meaning "That works!" or "That suits my schedule!"