It is a number from the first digit on the left that is not 0 to the last digit (including 0, and the scientific counting method does not count the n power of 10), which is called an effective number. Simply put, removing all zeros in front of a number is a valid number.

For example: 0.0 109, the first two zeros are not significant numbers, and the following 109 is significant numbers (note that the middle zeros are also counted).

In 3. 109 * 105 (3. 109 multiplied by the fifth power of10), 3.109 are all valid numbers, and the fifth power of10 is not.

5200000000, all valid numbers.

0.0230, the first two zeros are not valid numbers, and the last 230 is valid numbers (the last two zeros are also counted).

1.20 has three significant figures.

1 100.024 has seven significant digits.