The grammar structure A/V+(으)ㄹ 때 is used to express “when A/V” or “at the time of A/V”.
한국에 갔을 때 친구들을 많이 만났어요.
When I went to Korea, I met a lot of friends.
나는 처음 만났을 때부터 그녀를 좋아했어요.
I liked her from when we first met.
어릴 때부터 책을 정말 싫어했어요.
From when I was young I hated books.
서울에 살았을 때 한국어를 배웠어요.
When I lived in Seoul, I learnt Korean.