Sharps & flats

A sharp (♯) raises a note by a half step; a flat (♭) lowers it by a half step. The black key between C and D is therefore both C♯ (C raised) and D♭ (D lowered) — same key, two names.

Two notes that sound identical but are spelled differently are enharmonic. Which spelling you use depends on the key you are in — a detail that matters once we reach scales, but for now: C♯ and D♭ are the same sound.