These are notes that are named after a letter, such as A, B, C, etc…
The notes that fall in between Naturals are called Accidentals.
If the accidental is formed by raising a note up, we call it a Sharp (#).
For example, if you raise D up by one fret, it becomes D#.
For example, if you lower E by one fret, it becomes Eb.
If neither one is specified, just guess.