Gerund

When -ing forms are used like nouns, they are called gerunds. A gerund can be the subject, object or complement of a verb.

• Flying (subject) makes me sick.
• Trespassing (subject) is prohibited.
• Smoking (subject) is injurious to health.
• We don't allow smoking (object) in our house.
• I enjoy swimming (object) in the sea.
• My favourite activity is reading. (complement)
• The thought of failing never entered his head. (object of the preposition of)