In music notation, when a note falls outside of the established diatonic (or home) key of a song, it will be maked by a sharp (♯ - 1/2 step up), flat (♭- 1/2 step down,) or natural (♮) symbol. These symbols (♯,♭,♮) are also called accidentals.
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The key of C has no accidentals (you play it using only the white keys on a piano). If you were to play a note on the black keys while in the key of C, you'd be playing an accidental, and it would be notated by an accidental symbol.