A syllable is a part of a word that has one vowel sound.
People learn about syllables when they are learning to read and write.
Examples:
The word 'apple' has two syllables.
My name has one syllable.
The word 'elephant' has three syllables.
I can clap out the syllables in my name.
The longest word in the world has a lot of syllables.
I can count the syllables in my favorite song.