Or double flats or double sharps, which lower or raise the note one whole step, respectively.
(Rat feels his brain have a hiccup as he tries to grasp this information.)
Ummm....if you're going to change a note by a whole step, why not simply, y'know, write the note to be one step higher or lower in the first place? It's been more than two decades since I played in my high school's band, but as I recall, sheet-music staffs do allow for that.
As someone who's played Chopin's Waltz in C-Sharp Minor, I have the answer.
See, the key of c-sharp minor has F-sharp AND G-sharp included in the key signature. Without the double-sharp symbol, playing G-sharp and G-natural in the same measure would require 2 different accidentals, making the measure take longer to read and making practicing your Chopin even harder than it already is.
By labeling the G-natural as an F-double-sharp, you can follow up with a G-sharp without confusing the fuck out of people who are trying to read your sheet music.