The significant figures (also called significant digits) of a number are those digits that carry meaning contributing to its precision. This includes all digits except:
leading and trailing zeros which are merely placeholders to indicate the scale of the number.
spurious digits introduced, for example, by calculations carried out to greater accuracy than that of the original data, or measurements reported to a greater precision than the equipment supports.
Read more on – Wikipedia
Video on Significant Figures
Credit Khan Academy