Question

What are some characteristics of medieval music?

Answer 1

Think, Lord of the Rings c:

Answer 2

brass!

Answer 3

The easiest way to think of modes is as scales based only on "white notes" of the piano. They all sound different, just as the scale of A minor sounds different from C major. There are seven white notes, A, B, C, D, E, F, G, and each is the basis for a different mode.
The issue is complicated by the fact that the namings of modes changed, and it is also complicated by the fact that the same mode could get different names depending on how it was used.
The modes were called by the names of groups of people known to the ancient Greeks, with the names Lydian, Phrygian, Dorian, Mixolidian, and Locrian, but also Hypolydian and Hypophrigian. These were later changed to include Ionian and Aeolean instead of Hypolydian and Hypodorian, but that came after the Middle Ages
There is a link below to an article on musical modes, where more information can be found.

The reason the modes were used instead of the modern scales, which we seem to regard as so intuitively obvious, is that the ancient and medieval people used integer ratios to divide their scales, and when an instrument is tuned this way to a particular scale, the other scales all have notes that sound out of tune. The change to well tempering required mathematically irrational divisions based in rations involving the twelfth root of two. This was first done in the well tempered scale.