In a tone language (tonal language), different tones (like in music, but not as many) will change the meaning of the words, even if the pronunciation of the word is the same otherwise. English and almost all other European languages are not tone languages at all.
In some languages, pitch accent is important. That means a word's meaning could be different, depending on which syllable is stressed. Examples include Swedish, Norwegian, Serbo-Croatian and Lithuanian. Japanese has a similar system.
An example of tone language: in Mandarin, ma ma ma ma (麻媽罵馬) means four different things. If we include numbers that mean the tones, it can be written like ma2 ma1 ma4 ma3, which means "the hemp's mother scolds the horse." Some Romanizations write each tone with a different spelling, so ma2 ma1 ma4 ma3 would be ma mha mah maa in Gwoyeu Romatzyh Romanization. Most use numbers or accent marks (mā má mǎ mà in Pinyin).
Some tones may sound alike to people who do not speak a tone language and are easy to forget. So tone is the most difficult part of learning a tone language for many people.
Tonal Markings [change]
Vietnamese and Pinyin use accents (diacritics) as the tone marks for the Latin script. Each marking defines an altered sound for the syllable. Most syllables only have one tone marking. But, the letters in the syllable can be altered by other special character/letter markings. So, more markings can be in each syllable. Syllables usually form one word in un-hyphenated compound words. Pinyin may have style differences which break from convention (in use)- because it is a demonstration language. Vietnamese is a national script which always follows the syllable marking style.