The gerund is the -ing form of the verb when it is used as a noun.
For example, the word driving is a gerund in this sentence:
But it is not in this sentence:
We use gerunds when the verb is immediately after prepositions, e.g. of, against, after, for, in, about, etc.
We use gerunds when we are talking about an action or activity in a general way.
a) with nouns, e.g. thought, hope, idea, principle, difficulty:
b) with adjectives, e.g. tired of, fond of, angry at, etc.:
admit, appreciate, avoid, complete, consider, delay, deny, discuss, dislike, enjoy, finish, give up, can’t help, insist on, keep (on), look forward to, mind, miss, object to, practice, prefer, recall, recommend, risk, suggest, (get) used to, understand