what is the meaning of reliable digits and what are uncertain digits

In a number, if there are x no. of significant digits, then first (x-1) digits are reliable or certain digits and the last digit which is the estimated one is the uncertain digit.

 

For eg., if a distance is measured to be 738.4 m, it has four significant digits. The digits 7, 3 and 8 are reliable or certain, while the digit 4 is uncertain.