A gerund is a special form of a verb known as a verbal. Gerunds are created by starting with an action verb and adding -ing to the end. For example, "jump" is an action verb. The sentence "Jumping is good for your heart" uses the gerund form of "jump" as the subject of the sentence. Once the gerund is created, it can be used in any typical ways that nouns are used. For instance, a gerund can be the subject, object or predicate noun of a sentence.
Gerund phrases are groups of words surrounding a gerund, which together take on the role of a noun in the sentence. The gerund is the first word in the phrase. In the sentence "Being a teacher is an honor," the phrase "being a teacher" serves as the subject of the sentence. Another example would be "My favorite pastime is wandering around the mall." In this case, "wandering around the mall" is the predicate noun based on the gerund wandering. A gerund phrase means that a verb is being used in the role of a noun in a particular sentence.