People going into a Christian church expect to see stained glass windows. The version we now have was a twelfth-century invention.
They didn't have the big sheets of plate glass in the Middle Ages that we now take for granted, so if you look at medieval glass, you will see that it is a series of fairly small pieces, with leading in between. The different pieces would be stained different colors, and the glass painted in addition. It was rare for medieval stained glass just to be done in decorative patterns; rather, the windows would tell stories. Most common were Bible stories, followed by lives of the saints.
One has to assume that medieval people were far-sighted by our standards. Modern people often need binoculars to get a good look at the details of stained glass, but medieval people apparently would be told the stories with someone pointing out the images that were meant to illustrate the various scenes.
The thirteenth century was the great century for stained glass. The technique had been fully mastered, and the colors achieved were exquisite. For many years modern artisans were unsuccessful in reproducing the blue of thirteenth-century stained glass, an intense, beautiful shade, celebrated especially at the cathedral of Chartres. The modern artisans speculated that the secret might be "let it age for 700 years." Recently, however, they have discovered that certain chemical impurities in the sand that went into the glass was the answer, and occasionally a modern church can come close to the colors of medieval glass.
The image above is from a window at Chartres.
The greatest achievement of stained glass workmanship was the Sainte-Chapelle, located in Paris, not far from Notre Dame. Many tourists unfortunately miss it, because it is accessed through the modern Palais de Justice (national headquarters for the police), so you may pass handcuffed criminals while getting in line for the Sainte-Chapelle.
It was built by Louis IX (St. Louis) in the middle of the thirteenth century, as a suitable church for the Crown of Thorns, which had been "located" in the Holy Land and brought to France. Don't worry too much about this. Enjoy the glass. The building is essentially built of glass, with only narrow panels of actual wall between the stunning windows. During World War II the windows were disassembled and stored in the basement, to protect them from bombing, and there is a lively scholarly debate whether they were all put back in the right order.
A lot of medieval glass is unfortunately gone forever, smashed during the sixteenth-century wars of religion, the eighteenth-century French Revolution, and two world wars. Notre Dame in Paris has very little of its original medieval stained glass, due to seventeenth-century priests of the church, who decided it was "old fashioned" and smashed it out, to replace it with "modern" clear glass.
And then there's modern pollution, which eats away glass. Careful cleaning projects are being carried out. The grime and badly-pitted surface are cautiously removed, and then the window is covered with plexiglass, separated from the medieval glass by a layer of mineral oil. It seems to work pretty well.