This symbol is called a sharp sign.
When a sharp note is written on the staff, it is placed before the note.
The F sharp (F#) note is shown here, with the sharp sign (#) placed before it.
To play the note F#, use the thumb, first, second and third fingers of your left hand, and the second and third fingers of your right hand. Make sure you cover both holes with the third finger of your right hand.
Instead of writing a sharp sign before every F note, it is easier to write just one sharp sign after the treble clef. This means that all F notes on the staff are played as F#, even though there is no sharp sign placed before the note.