Sharps and flats are special symbols in music that tell you to play a note one-half step higher or lower than it is written. They are often played using the black keys on the piano.

A sharp tells you to play a note one-half step higher than it is written.
A flat tells you to play a note one-half step lower than it is written.

The sharp sign is very similar to the number sign, pound symbol, or hashtag. The main difference is that the sharp sign has slanted parallel lines and straight vertical lines.
The flat sign looks like a lower case b except the bottom is less round.