Question

What are significant figures?

Answer 1

each of the digits of a number that are used to express it to the required degree of accuracy, starting from the first nonzero digit.

Answer 2

Other number than zero are always significant.

Zero in the end of integer is usually non-significant.
E.g 100 has one significant number
100 m run in competition has 3 significant numbers

Zeros at the beginning of decimal number are not significant.
0.0125m has 3 significant number

Zero in the end of decimal number is significant.
0.540 g has three significant number.