Relaxing with my friends is fun.
Sarah likes reading funny books.
A gerund is a verb +ing that functions as a noun in a sentence.
A gerund can be used as the subject of a sentence or as an object.
after enjoy, finish, imagine, miss, practice --> you need a gerund
e.g. Tom can't imagine living in New York.
after begin/start, continue, hate, like, love, prefer --> you can use a gerund OR the to-infinitive
e.g. I love eating Thai food. I love to eat Thai food.
after would like, would love, would hate --> you can only use the to-infinitive
e.g. Tom would hate to live in New York.
after the preposition after, without, for, of, ... --> you need a gerund
e.g. You can only live for a few days without drinking.