Significant figures
Your answer cannot be more precise than the least precise number you started with.
A worked example
4.5 x 2.11 = 9.495 on a calculator. 4.5 has 2 significant figures and 2.11 has 3, so the answer keeps 2: 9.5.
The mistake people actually make
Count from the first non-zero digit. In 0.0043 there are 2 significant figures, not 5.