Picture a small British town with one street running through the middle of it. The bank, the baker, the chemist and two cafés are all on that street, and everybody walks down it at some point in the week. That one street is the high street. And because so much of the town's buying and selling happens along it, its name slowly came to stand for the buying and selling itself.
From the word’s history
Wiktionary traces high street to Old English hēahstrǣt, a high road — high + street. The high here can be thought of as chief or main, the same high as in high road, rather than anything to do with height.
Wiktionary 把 high street 追到古英文 hēahstrǣt（高的路），也就是 high 加 street。這裡的 high 可以想成「主要的、幹線的」，跟 high road 是同一個用法，不是在講高度。
Wiktionary, entry “high street”: “From Middle English high strete … from Old English hēahstrǣt (‘high road’), equivalent to high + street.”