The Greeks believed that the source of variety in expression in music is how the intervals are arranged. An interval is the space between two notes. For example, the interval between an adjacent white and black key on a piano is a half step or between two white keys is a whole step. They based their musical modes on a desirable arrangement of intervals. The technical aspect to music theory is modes .
Modes are an eight note scale that is ascending .
Ascending means going up. The modes are made of white keys on the piano. Of course they didn't have pianos in ancient Greece. Using white keys on the piano today produces the same intervals as in the Greek modes but makes it easier to explain in modern terms. There were seven modes.