This is a sharp sign.
When a sharp sign is placed before a note on the staff, it means that you play the key immediately to its right. This key may be either black or white. E.g., the note F sharp (written as F#) is shown on the staff, as follows.
To play the note F#, play the black key immediately to the right of the F note (white key), as shown on the diagram.
Instead of writing a sharp sign before every F# on the staff, it is easier to write just one sharp sign after the treble clef. This means that all the F notes on the staff are played as F#, even though there is no sharp sign placed before them.