Say you have a number like 12.3458.
Rounding to the nearest decimal means giving the closest possible number with a tenth precision.
Basically, here's what you do :
if the number after the tenth is between 0 and 4, just truncate to the tenth. For instance, 12.3458 -> 12.3
If it's between 5 and 9, truncate to the tenth and add 0.1 : for instance, 256.842351 -> 256.9
Or, 23.9762 -> 24.0
