What does a flat do?

A sharp raises a note, while a flat lowers a note by half step. Accidentals in a measure last throughout the measure but can be canceled by a natural sign. Sharps and flats share a common pitch and are called enharmonic.

Explanation:

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Lowers by half step

Explanation: A flat sign means to play the key that is a half step lower, therefore if you come across an F note on a piece of music, it means to play the note that is a half step lower. That note is F flat and it is the enharmonic of E on piano, a white key.