You can think of harmonize as two voices singing notes that are not the same. One carries the tune; the other sits above or below it. They stay different, yet the two lines lean on each other and come out as one rounder sound. Every other use of the word can be read off that picture: keep the parts different, choose them so they fit, and the whole thing stops sounding like noise.
From the word’s history
Wiktionary gives harmonize as from French harmoniser, and by surface analysis harmony + -ize, that is, to make things into harmony. The musical sense sits closest to that literal build.
Wiktionary 記載 harmonize 來自法文 harmoniser，就字面拆解則是 harmony（和聲、和諧）加上 -ize（使變成…），也就是「使之成為和諧」。音樂上的用法最貼近這個字面結構，其他用法讀起來像是從這裡借出去的。
Wiktionary entry for harmonize (packet etymology_text): From French harmoniser. By surface analysis, harmony + -ize.
登入就可以看每一支怎麼連回核心畫面。