This answer is written on the assumption that you are looking for medieval or Middle English (11th to 15th century) examples of what is often referred to as 'breaking the fourth wall'. Breaking the fourth wall is when the narrator, author, or even a character within a work, address the reader of the story directly. For example:
Roy, the hero of this tale, was indeed an idiot. But you know that by
now, don't you dear reader?
In the above example, the narrator is clearly addressing the reader. It can give the reader the feeling that they aren't reading the story as from a first hand point of view, but rather second hand. It reminds me of the way there was a fad for writing novels in the form of a series of letters or diary entries for a while. It feels more intimate, and I quite enjoy it when it's done well.
I've also often seen this tool used for comedic effect or in children's novels, but that isn't always the case. Sometimes it can be used as a kind of content warning where the narrator, author, or a character warns the reader that something unpleasant is about to happen and that they should skip ahead if they should wish to avoid it, or even stop reading at that point. I know that Stephen King used something similar in the end of his Dark Tower series, for example.
As for whether breaking the fourth wall was common in medieval literature, I can think of two examples off the top of my head, and that's The Canterbury Tales by Geoffrey Chaucer, and The Divine Comedy by Dante Alighieri. I can't remember which tale of The Canterbury Tales it was, but I believe Chaucer used this trope as a content warning, telling the reader to choose another story to read if they felt unhappy with the upcoming one. For The Divine Comedy, there's too many examples to list. Because Dante himself was the main character, he often addressed the reader directly about what was happening. So, while I can't say whether it was a commonly used writing technique, it at least existed as one.
So, to sum up, there are some examples of this trope being used in medieval literature, and is still used somewhat today. I, personally, think it can give the work an intimate feeling that some readers I'm sure would enjoy (if done well).