It is possible to give exact numbers for some measurements made, for example, one can say that a man has 5 fingers in each hand and a total of 10 fingers or that there are 16 balls in kept in a box. But for most measurements on the other hand it is not possible to give an exact value of the measurement. The accuracy of the value that one gets is dependent on the instrument being used. For example if one were to measure length using a scale that only has lines denoting centimeters, it is not possible to measure a length to an accuracy of millimeters.
Significant figures are used to indicate the accuracy of the instrument that is being used to make a measurement. When a measurement is indicated, the leading zeros and any trailing zeros in the absence of a decimal point are not significant. Embedded zeros in the measurement are significant. The easiest way to determine the significant numbers is by using the scientific notation.