The before a noun shows that what is referred to is already known to the speaker, listener, writer and/or reader (it is the definite article) ... Cambridge Dictionary
There are a lot of examples when the noun is already known to the listener:
I watched the movie you recommended.
Both of them (speaker and listener) know the movie, The listener know it because he recommended it.
Also there are a lot of examples in reader case:
Once when a lion was asleep, a little mouse began running up and down on him. This soon awakened the lion ...
The lion mentioned before so the writer and the reader know which lion.
My question: What the dictionary mean by it is already known to the speaker or writer? It would be great if there is some examples in these cases.