Significant figures are what we call all those digits in a number that are known for sure (or for which there is a some certainty).

In the measurement expressed as 4.563 m, if we know up to the 4th digit with certainty, this tells us that the instrument which measured this length can register up to millimetres. This measurement has four significant figures.

You can practise with the following visual; you just have to click the init control as many times as you like and a new exercise will appear.