We normally round off numbers from calculations that have excessive decimal places.
Rounding off works by looking at the next digit along from where you want to round off.
When approximating answers, we can either do so to a certain number of decimal places (d.p.) or significant figures (s.f.).
The number of d.p. is simply the number of digits that come after the decimal point.
The number of significant figures is the number of digits that have importance to the number's value.
We should normally give answers to 2 or 3 d.p. unless asked otherwise.
To avoid confusion on whether or not zeros are significant in large numbers, it's a good idea to state the number clearly in brackets.