Significant digits are a way of describing the precision of measurements in a scale-independent way.
Typically if a number is in normal scientific notation, the number of significant digits is the length (number of digits) of the mantissa or significand. The significant figures of a number are those digits that carry meaning contributing to its precision. This includes all digits except: All leading zeros, Trailing zeros when they are merely placeholders to indicate the scale of the number (exact rules are explained at Identifying significant figures), and spurious digits introduced, for example, by calculations carried out to greater precision than that of the original data, or measurements reported to a greater precision than the equipment supports.
Numbers are often rounded to avoid reporting insignificant figures. For instance, if a device measures to the nearest gram and gives a reading of 12.345 kg, it would create false precision to express this measurement as 12.34500 kg. Significant Figures - Wikipedia
