theme
The main melody that a piece is built on.
variation
A version of the theme that changes some elements but keeps the theme recognizable.
theme and variations
A musical form: the theme is played first, then a series of variations (A, A1, A2, A3...).
repeat sign
A double bar line with two dots. It tells you to go back and play a section again.
first and second endings
Brackets marked 1. and 2. over the end of a repeated section. Play the 1st ending the first time, then skip it and play the 2nd ending after the repeat.
mode (major/minor)
Whether music uses a major scale (often sounds bright) or a minor scale (often sounds darker).