Each black key has two different names based on the white keys it is next to.
If we think of it as a note that has been raised, we call it Sharp, written #.
If we think of it as a note that has been lowered, we call it Flat, written b.
Whether it is better to call it sharp or flat is something we will learn later.
Any alteration of a note, sharp or flat, is called an Accidental.