A participial phrase is a group of words that includes a participle, which is a verb form that can function as an adjective. These phrases provide additional information about a noun or pronoun in a sentence. For example, in the sentence "The cat, sleeping on the windowsill, looks peaceful," the phrase describes the cat. Participial phrases can begin with present participles (ending in -ing) or past participles (often ending in -ed or -en). They help to create more complex and descriptive sentences. For instance, "The broken vase lay on the floor" uses a past participial phrase to give more detail about the vase.