Gerund are nouns and they are formed by adding -ing onto a verb. Take a look at this paragraph - the gerunds are in red. Ex : Learn - Learning / Speak - Speaking / Listen - Listening.

1. As a subject - Walking is a good form of exercise
2. As a direct object - Some people like swimming to keep fit.
3. Objects of prepositions - Another method of learning English is to read lots of fiction books.