The closed syllable rule refers to the need for syllables with a single short vowel to be followed, or closed, by a consonant. Absent the closing consonant, the vowel becomes long.
The word "mitten" provides an example of the rule. Upon hearing the word, someone without knowledge of the rule, either consciously or innately, might spell the word "miten." However, this spelling would result in the first syllable "mi," which in written English creates a long vowel. To create the short "i" sound, the closed syllable rule dictates closing of the syllable, thus creating the syllable "mit."
Exceptions to the rule consist of all syllables that end in "-old," "-ild" or -"olt," as well as most syllables ending with "-ind" or "-ost."