The document explains the grammar rule of doubling the final consonant when adding suffixes like "-ed" to words. It notes that this is done to prevent the vowel sound from changing. It provides examples like "hop+ped" and explains you double the final consonant so the suffix "-ed" can't override the original vowel sound. It then prompts the reader to apply this rule to example words ending in single consonants by adding the doubled consonant before "-ed".