What does uppercase letters mean?

These are lowercase letters : abcdefghijklmnopqrstuvwxyz

These are uppercase letters: ABCDEFGHIJKLMNOPQRSTUVWXYZ

They are called "uppercase" and "lowercase" because in the days when printing was done using lead type, the printer compiling the text kept the capital letters in the "upper row" and the small letters in the "lower row" so that they were sorted out ready to hand when needed.