Today the ability to read and write is essential for most members of the society and is taught as part of even the most basic education. People who are illiterate have difficulties with participating in much of the modern world.
My layman historical knowledge assumes the typical European farmer in the Middle Ages could not read and write much besides his own name.
What changed and when?
Edit: Can't edit the title. Therefore, please ignore typos and grammar mistakes in this post about literacy ;)
u/Celebreth answered What was literacy like in Ancient times? which might help to tide you over until someone takes a swing at this question specifically.
Happy reading!