Key signatures refer to the group of sharps or flats shown at the beginning of sheet music - and usually on each line of the page.
The sharp keys are these:
Notice: the major key is always a semitone higher than the last sharp.
And the flat keys are these:
Notice: the major key is always the second-last flat.
The last key signatures of F sharp and G flat are enharmonic - using the same keys called by different names. And you may see 7 flats or sharps too, though this is rare!