This is a sharp sign.
When a sharp note is written on the staff, the sharp sign is placed before the note.
Here is an example which makes use of the note F#. The sharp sign affects all F notes within the bar in which it appears. Eg: in bars 1, 2 and 3 the # sign appears before the first F in each bar. The sharp also applies to all other F notes within that particular bar. The effect of a sharp sign is cancelled by a bar line, meaning that a new sharp sign is needed to indicate a new F# note in the following bar.