The document discusses the gerund, a non-finite verb form ending in "-ing" that functions as a noun. Gerunds can serve as the subject of a sentence, follow prepositions, or come after phrasal verbs. To form a gerund, typically add "-ing" to the base verb form, though there are rules for verbs ending in "e" or "ie" and one-syllable verbs with consonant-vowel-consonant endings.