Examples:
* Swimming is my favorite hobby. (Swimming is the gerund, acting as the subject of the sentence.)
* I enjoy reading. (Reading is the gerund, acting as the object of the verb "enjoy".)
* She is good at dancing. (Dancing is the gerund, acting as a complement to the adjective "good".)
Gerunds always end in "-ing" and function as nouns in a sentence. They can be the subject, object, or complement of a verb or preposition.