A significant figure is one that is measured rather than “guessed”. If you say that length is “approximately” 100m, then that is only 1 significant figure. If you measure accurately 100m for a race than that is 3 significant figures. If you are given the number 100 without comment, it could be 1,2 or 3 significant figures! The number 1.01 has 3 significant figures. The number 0.001 has only 1 significant figure. I hope this helps.