A waltz is a piece of music in three time. Originally all waltzes were written as dances, but in the nineteenth century composers starting writing waltzes that were intended for listening only. The accompaniment to a waltz often has a strong first beat and two matching weaker second beats (it sounds like oom-pah-pah).
There are many famous waltzes. A good example is Johann Strauss’s Blue Danube waltz.
https://www.youtube.com/watch?v=eYFhLM71kOY
Can you count the three beats in a bar as you are listening? This is quite a fast version.
See what other examples of waltzes you can find and listen to some.
Find out more about the waltz as a dance. Perhaps you could learn to dance a waltz or watch some experts dancing one.