In some instances, you'll find yourself needing to catch readers up to speed.
We see this often in worldbuilding, which might involve characters preparing for an annual tradition or explaining why certain things are as they are. Prologues might clue a reader into important things that happened before the story kicks off. In sequels, the first chapter might include a brief recap of the previous book to refresh the reader's memory while setting up things yet to come.
Orienting your reader in the world of your story and filling them in on what your characters are doing makes it easier for them to immerse themselves in the adventure. If left to unscramble terminology or struggle to make sense of a character's everyday life before the inciting incident occurs, it's possible readers won't be able to find their way.
Some methods are better than others, and among them is a character saying, "As you know."
There are times this phrase can work, but it can also be a signal of an infodump.
It's a way of telling the reader that they're about to get a bunch of information handed to them. While this may let them know they really need to pay attention to these details because they are important, there is also the chance it disrupts the pace of the narration.
Think of it like tuning into your favorite TV show. Before the new episode begins, there might be a recap of the previous week in order to refresh your memory or catch up anyone who missed it. This segment typically starts with a voice-over saying, "Previously on..." or "Last week on..." before a montage of clips highlighting key moments.
"As you know" can have a similar effect on your reader. By calling attention to the catch-up, you may risk breaking up the flow of your narration.
Take that above example but instead picture yourself binging the episodes back-to-back via a streaming platform. You don't necessarily need the recap if you're watching the show in such a fashion, so these "Previously On" montages might be accompanied by a skip button if they're not glossed over automatically. In either case, you're able to continue without interruption.
Having "On the last episode of..." has the potential to take me out of the moment when I'm binging episodes, and "As you know" can have the same effect on your readers if used in your book.
Following the above example of a TV recap, when you're invested in a story, the last thing you would want is to be interrupted. "As you know" can break the flow because it calls attention to the exposition as opposed to gradually doling out that information.
"As You Know" also comes with a sense of redundancy in-universe. When a character uses this line, it can feel as if they are saying, "You know this but the reader doesn't."
Additionally, if the character already knows what is about to said, reiterating it can seem unnecessary. Speaking from experience, "As you know" conversations tend to be among the things that end up on the chopping block in the first round of edits because of the redundancy.
Exposition can be tricky. There's information that needs to be laid out in order for your reader to understand the characters' actions and why things in your story's world are as they are. You likely have an encyclopedic knowledge of your WIP that your readers don't. They need to be filled in, but in a way that doesn't overwhelm them or stand out in the narration.
By drawing more attention to the exposition, you might be making it harder to digest.
"As you know" is perhaps the most common example, but it's not the only form of this infodumping you'll encounter in fiction.
Villains, especially those on the cheesy and cliche side, are often renowned for grandiose speeches in which they explain their plans and motivations behind them. This is particularly associated with moments in which the protagonist finds themselves trapped or at a disadvantage. Taunting their foe may be for their own enjoyment, but it's also a way for the author to tell the reader everything they need to know about the conflict and stakes.
Broadcast media is another popular tool for this purpose. There might be a TV on in a diner or bar that is interrupted by a news bulletin. The character might also happen to tune into the radio at just the right time to coincidentally hear a report on the scene in a crime novel or an alert from the emergency broadcast system in a story set during a zombie apocalypse.
"This just in" or "If you're just joining us" can function similarly to "As you know."
Phrases and occasions such as these can make it easier to pass this info along, but not without downsides.
Handling exposition is a post for another day—quite frankly, it's probably several posts for another day—but suffice it to say that it's typically better to weave it in gradually, as it becomes relevant to the characters' experiences.
And that's what you need to know about "As you know."