Each unique vowel sound in a word typically equates to one syllable. Syllables are found in all spoken languages and the specific rules of syllables can vary from language to language.
The structure of a syllable can be broken into three parts: the onset, the nucleus, and then the coda. The nucleus is the syllable’s unique vowel sound. The onset and coda are the constants that precede and follow the nucleus, respectively. A syllable always has a nucleus, but may or may not have an onset or a coda.
For example, the syllable “up" has no onset; the letter “u” is the nucleus, and the letter “p” is the coda. Likewise, in “tree," the consonant cluster “tr” forms the onset, “ee” forms the nucleus, and there is no coda.