Melody Credits are credits offered during a 1-month free trial to use for creating a melody.
During the free trial, you get thirty melody credits. And one credit is used per melody.
When using the Auto Melody function, it creates three melodies at once and uses three credits.
You can see the balance of credits and used credits in the credit bar each time you use them, as shown in the image below.
You can check the balance of credits in the upper right of the melody section on the MUSIA plugin, as well as you can find them on the profile settings page on the website.