Gerunds are one of three kinds of verbals. Verbals are verbs that function as another part of speech. The gerund always functions as a noun.
A gerund always ends in ing. It also always functions as a noun. This means that all gerunds will act as subjects of a sentence, objects of prepositions, predicate nouns, or direct objects.
Bertha enjoys hiking.
Hiking is the gerund. In this position, it functions as the direct object of enjoys.
Jogging is a healthy activity.
Jogging is the gerund. It ends in ing as all gerunds do. Here, it functions as the subject of the sentence.
Arnold went to jail for stealing.
Stealing is the gerund, and here it acts as the object of the preposition for.
Seeing is believing.
here are two gerunds in this short sentence. Believing is a gerund acting as a predicate noun.