Typically used in the past tense, attaching 기로 하다 to a verb stem expresses the making of a promise to yourself or to another.
올해 한국에 가기로 했어요.
I’ve decided to go to Korea this coming year.
내일 남자친구와 데이트하기로 했어요.
Tomorrow my boyfriend and I are going to go on a date.
이번 주말에 열심히 공부하기로 했어요.
I’ve decided to study hard this weekend.