Four flats at beginning of sheet music?

I currently want to learn this song on piano, and I have the sheet
music. I can read music fairly well since music was a subject to be
taught from Kinder- Year 8, but we never learnt what to do when there
are flats or sharps after the treble clef at the beginning of the
song. In my sheet music here there are 4 flats for both the treble and
bass clef sections. I'm not sure what the proper name for teh 4 flats
is obviously. What will this do to the notes? If you could give me a
site or something that explains it well, or if you are happy enough to
give up some of your time to explain it here, that woudl be great.
Thanks