articles - "The" before a noun shows that what is referred to is already known to the speaker

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.

Answer

The can indicate to the hearer/reader that the speaker/writer has a specific instance in mind which she is about to define. For instance:

I just saw the woman who wrote that article.

Here the speaker knows which woman she's talking about, but the hearer doesn't know which woman is meant until he hears who wrote that article.

And in fiction it is quite common for an author to introduce characters or situations with the as if the reader had already encountered them. For instance, Virginia Woolf's story "The Window" begins like this:

"Yes, of course, if it's fine tomorrow," said Mrs. Ramsay. "But you'll have to be up with the lark," she added.

To her son these words conveyed an extraordinary joy, as if it were settled, the expedition were bound to take place, and the wonder to which he had looked forward, for years and years it seemed, was, after a night's darkness and a day's sail, within touch.

Here the son knows what the expedition and the wonder are, and of course Woolf does too; but the reader doesn't. This has the effect of putting the reader in the middle of things and excites his desire to find out what the expedition and the wonder are.