Rudiment basically means the basics of something or the foundation of something, something that is essential to learning something else.
For example, in math a rudiment would be learning addition or in reading a rudiment would be learning the alphabet or to write, these are things we would need in order to progress.
In Music the Rudiment or the basic things you need to know would be:
The Music Notes (clefs,whole note,half note, quarter note, eighth note,etc)
