A closed syllable ends with one or more consonant sounds after the vowel, so the vowel is usually short in English pronunciation: cat, in, tend. In poetry, closed syllables matter when counting syllables, reading meter, or deciding how a word should sound in a line.
They are often contrasted with open syllables, which end in a vowel sound.
Example
The crisp closed syllables in "slant," "stone," and "drift" can make a line sound firm and clipped:
Cold stone drifts beneath the slant of dusk.