A gerund is a verb form that ends in -ing and functions as a noun in a sentence. For example, in the sentence "Swimming is fun," the word "swimming" acts as the subject, making it a gerund. Gerunds can also be used as objects, such as in "I enjoy reading," where "reading" is the object of the verb "enjoy." Gerunds can be part of phrases, known as gerund phrases, which include the gerund and any modifiers or objects. For instance, in "Running in the park is refreshing," the phrase "Running in the park" serves as the subject. Gerunds are versatile and can appear in various grammatical roles, enhancing sentence structure.