First, let's talk about books. A literate person needed something to read. Bound printed books, as we know them today, didn't exist back then. Remember, the printing press wasn't created by Gutenberg until around 1440. Before then, books were created by hand by copying an existing book. Entire monasteries dedicated their attention to hand-copying books page after page. Many of these books were Holy books such as the Bible. Because the creation of a book was so time-intensive, the resulting book was extremely expensive. Typically only the most wealthy of people would have a book.
While in Roman times people would read from scrolls of parchment, by the Middle Ages a standard form was the "codex" - a series of pages connected at the back / spine. In essence a codex is the shape of our current books - just the pages were written out by hand. The use of paper (wood pulp) began to replace parchment (animal skin) - paper was cheaper and this also helped books become a bit more accessible.
Because paper and parchment were relatively expensive, people would never use those for note taking. Instead, they would use wax tablets. Wax tablets have been used since nearly the beginning of civilization and were even used as recently as the 1800s. They have been a standard method of note taking for many different civilizations. The benefit of a wax tablet was that wax was easy to get, you could write whatever you wished with a stylus, and then simply mush the wax down when you were done to start afresh. You could write words, numbers, draw shapes, whatever you wished. Here's a great article all about how heavily the wax tablet was used in medieval times - https://www.bl.uk/eblj/1994articles/pdf/article1.pdf
And here's an image of a woman in Pompeii using a wax tablet, long before Medieval times.
So with these various options available, who would use them?
Sure, monks and nuns. They were dedicated to preserving their Holy words. But it was not just the monks and nuns who knew how to read and write. There was a slew of civil servants who had to keep track of the taxes. Singers who needed to read song lyrics. Poets. Those who managed guilds. Those who managed noble households. Generals distributing orders to their troops. Rulers distributing orders to their ruled. Many of these people focused on Latin, which was a common language across many countries, but there were also those who learned a local language as well.
Yes, there are all the tales of taverns having picture-signs like a Black Kettle rather than words, and it's true that a portion of the population couldn't read. But it did not mean that everyone was illiterate. Graffiti shows us that even hooligans often knew how to write quite well :).
Soldiers wrote letters home. Religious-minded people wrote prayers and sacred songs. Household members kept notes on supplies. And there were all sorts of people who couldn't write for themselves but who could read what other people wrote. They weren't quite skilled enough to figure out how to form letters and sentences, but they could puzzle out one which was given to them.
Still, it's fair to say that much of this depended on the particular environment one was raised in. If a person was born into a fisherman's hut and the entire village was made up of simple fishermen, who all clustered together at the edge of the ocean, it could be that none of them were literate. They knew to make marks for numbers, and perhaps a symbol for the type of fish, and that would be it. That would be what they needed to get them through the day.
But if someone grew up in a guild household with assets, and it was expected they were to keep track of sales and inventory, children could be trained to read as soon as possible so they could become useful. Remember, children in this time period weren't thought of as cute playful things. They were thought of as young adults who needed to pull their weight. They were taught whatever tasks possible as soon as they could learn. Both for family survival and also to help them grow up to be functional adults.
Since women often "ran the household" in these times while men handled the defense and out-of-household tasks, it was natural that many of these women needed to read and write in order to manage inventories, keep lists, and do the myriad of other tasks required.
You can even buy a wax tablet of your very own, from Amazon, to see how they work!
Historic Wax Tablet (replica) from Amazon
Medieval Romance Basics
Medieval Romance for Villagers
Medieval Romance for Nobles
Medieval Romance for Men
Medieval Romance for Women
Life in Medieval Days
Lisa's Medieval Romances
Seeking the Truth
A Sense of Duty
Online Literary Magazines
Lisa Shea's Homepage