Used to relate something that the speaker came to know through direct past experience. Usually conjugated in present tense. Often used as '-더라' in casual speech.
Example
일본에도착하니까눈이왔더라고요.
When I arrived in Japan, it was snowing.
이제품이요즘청소년들에게정말인기가많더라고요.
This product is really popular among young people these days.
민주가축구되게잘하더라.
Minjoo is seriously good at soccer.
한국에처음왔을때외국인이별로없더라고요.
There weren't many foreigners in Korea when I first came.
2.
Confirming a forgotten experience
who/where/when was it again?
Used in casual speech to wonder to oneself while recalling a past event, or to ask the listener about a shared past experience. 뭐더라 / 뭐였더라 can be filler words.