In this construction, N1 is a topic and the predicate tells something about N2, which is part of N1. N1 is a whole and N2 is its part. The same meaning can be expressed by the construction "N1の N2は ～".
象は 鼻が 長いです。
Speaking of elephants, their trunks are long./Elephants have long trunks.
赤ちゃんは かおが 丸いです。
Babies have round faces.
ローラさんは 足が ほそいです。
Laura has slender legs.
この 切手は 色が きれいです。
This stamp is beautiful in color.
Lesson 9-9「この 町は せまい 道が 多いです。」
This town has a lot of narrow streets.
Lesson 14-3「わたしは おんがくが 好きです。」
I like music.
Lesson 14-4「ジョンさんは ダンスが 上手です。」
John is good at dancing.