Gerunds are verb forms ending in "-ing" that can be used as nouns. Gerunds can serve as the subject of a sentence, after certain verbs like "like" and "enjoy", and after prepositions like "in" and "before". Some examples of gerund usage include "running is good exercise", "I enjoy swimming", and "before going to bed". Gerunds are also used after the verb "go" to indicate activities, such as "we went hiking on Sunday".