A syllable is that part of the word that contains 1 vowel sound irrespective of the number of vowels.
For example, ‘cat’ has 1 syllable as it has one vowel sound /æ/ (monophthongs). Similarly, ‘deer’ has 1 syllable as it has one vowel sound / ɪə/ (diphthong).
‘Snake’ is also a 1-syllable word as it has one vowel sound / eɪ/ (diphthong)
‘Camel’ is a 2-syllable word as it has two vowel sounds /æ/ and /ə/
‘Crocodile’ is a 3-syllable word as it has three vowel sounds /ɒ/, /ə/ and /aɪ/
Notice that number of vowels doesn't decide the number of syllables in a word.
Why learn syllabication?
1. Better spelling
Understanding of syllables and syllabication helps us learn the spellings of big words faster.
2. Better pronunciation
Once we know that ‘lion’ is a 1-syllable word (/laɪn/), we will not pronounce it as laa-yun
3. Better grammar
The same word can be a noun or a verb depending on which syllable of the word is the stress point. For example, PRE-sent (prezənt) is a noun/adjective, while pre-SENT (prɪzent) is a verb
4. Better accent
English is all about stress points. Stress points in words and sentences are very important to communicate the speaker’s intentions effectively and correctly.
(Note for parents: If you often feel that while you can understand what your on-shore boss says, he doesn't understand what you say, then you should learn syllabication and stress points.)