Just like how we use letters to write words, ancient Greeks used special symbols and markings to write down music. These symbols and markings would tell the musician how high or low a note should be, how long it should be held, and how it should be played.
One of the most important symbols in ancient Greek musical notation is the "tetragrammaton," which looks like four lines stacked on top of each other. Each line represents a different pitch, or how high or low the note should be. Musicians would use these symbols to write down melodies, songs, and even entire musical performances.
Although we don't know exactly how these symbols and markings sounded, archaeologists and musicologists have been able to piece together some of the ancient Greek music by studying texts and artifacts. Some of the surviving pieces sound very different from the music we listen to today, with lots of strings and pipes, but some parts also sound similar to modern-day music.
In conclusion, ancient Greeks had their own unique way of writing down music using special symbols and markings, which would tell the musician how to play the melody. While we don't have a complete understanding of how it sounded, we're able to piece together parts of it through archaeological and musicological research.