This is the 2762nd most frequent Korean word.
왔던
"왔던" means "that came" or "had come" depending on the context.
Here, '왔던' indicates a past action of coming to a specific place that is being referenced.
그 곳은 내가 예전에 왔던 곳이에요.
That is the place I came to before.
In this sentence, '왔던' depicts a past visit by the friend to a location being discussed.
친구가 전에 왔던 이야기를 들었습니다.
I heard a story about the place my friend came before.
In this context, '왔던' points to the past experience of visiting a specific restaurant and the related memory.