The earliest music in the Middle Ages was plainsong -a chanting of a single line of melody, which took the rhythm from the words. Gradually more parts were added, simply doubling the original plainsong melody at intervals of a fourth, fifth or an octave. This is called polyphony. A canon is anothe example of early polyphony. You have probably heard it called in the round. To illustrate this, you could have your students sing "Row, row, row your Boat," with each student starting at a different time. The polyphonic music of Machaut, who worked for King John of Bohemia, reflects the Gothic style, which is known for being highly ornamental, as compared to the plainsong of before.
Homegrown Learners has a great post with notebooking page on Medieval Music.