Significant Figures are the digits in a given number that are 'reliable' for calculation.
Leading Zeroes refer to zeroes that occur at the start of a number. For example, 0.0012030, the zeroes highlighted in red are leading zeroes. Leading zeroes are not significant.
Trailing Zeroes refer to zeroes that occur at the end of a number. For example, 1203000, the zeroes highlighted in red are trailing zeroes. Trailing zeroes may or may not be significant.
Spurious Digits (Not in Syllabus) are not significant.
Method
To round a number to 3 significant figures, we look at the 4th significant figure.
If it is less than 5, we round the 3rd significant figure down, otherwise we round it up.