The significant digits and its rules
The scientific instruments from the lab have a certain sensitivity expressing of values with a limited number of digits. The figures obtained from a measurement are called significant figures, as recorded by the instrument effectively.
The number of certain digits is given by those notes with some confidence and uncertain.
Here's a general definition: are significant digits a physical value the first digit uncertain and l ’ set of digits safe.
For example, 187,32 m is expressed with four digits of which the first four are safe and last is uncertain. This means that the real measure should be, with a suitable approximation between 187,31 and 187,33 m.
Here are the basic rules that allow us to represent the significant digits in the right way:

all nonnull values represent significant digits
example: 123456 has 6 significant digits 
zeros between nonzero numbers are significant digits
example: 4507003 to zeros are significant digits 
the zeros preceding the first significant digit are not significant digits
example: 0,00125 the zeros are not significant digits (This number has only three significant digits) 
trailing zeros are significant only if the comma
example; 14600 the zeros are not significant