People don’t generally speak in a monotone. Even someone who couldn’t carry a tune if it had a handle on it uses a different melody to ask a question than to make a statement, and in a sentence like “It was the first time I had even been there,” says “been” on a higher pitch than the rest of the words.
Still, if someone speaks in a monotone in English, other English-speakers can easily understand. But in many languages, pitch is as important as consonants and vowels for distinguishing one word from another. In English, “pay” and “bay” are different because they have different starting sounds. But imagine if “pay” said on a high pitch meant “to give money,” while “pay” said on a low pitch meant “a broad inlet of the sea where the land curves inward.” That’s what it feels like to speak what linguists call a tonal language. At least a billion and a half people worldwide do it their entire lives and think nothing of it.
Mandarin Chinese, with its four tones, is a typical example. Take the word ma. If you say it the way an English-speaker would say it, just reading it sitting by itself on a page, then it means “scold.” Say ma as if you were looking for your mother—ma?—and it means “rough.” If you were just whining at her—“ma-a-a?!?”—with your voice swooping down a bit and then back up even higher, that would mean, believe it or not, “horse.” And if you say ma on a high pitch, as if you were singing the first syllable of “The Star-Spangled Banner” as ma instead of “oh” for some reason, that would actually mean mother. That’s the way almost every syllable works in Chinese.