With this key you'll be able to change the sign of your input or the result of a calculation.
Say you input 325 and ment to put in -325. Just hit the +/- key. It toggles the value from 325 to -325 (and back when hitting +/- once again).
Same thing happens with the answer of your calculation. Just try 20+30 = 50
Then hit +/- to make it -50
