significant figures or "sig figs" describe the accuracy of your result. For example, if I was to measure a table with a ruler I might say that it had a width of 62.5 cm and length of 212.7 cm. I wouldn't say that it had a width of 62.50000 cm, because that implies that I know it isn't 62.50005 cm or something. If I as then to calculate the area of the table top, I say that it was 1.33 m^2, not 1.329375 m^2 - because the numbers I was using to calculate were only accurate to 3 and 4 significant figures respectively.

examples

In 2.00 there are 3 sig figs

In 2.0 there are 2 sig figs

In 0.0001 there is 1 sig fig

In 0.00010 there are 2 sig figs

In 0.00101 there are 3 sig figs

In 100 there is 1 sig fig (could also be 3 - this is one of the reason why scientific notation is good, it avoids this ambiguity)

In 100.0 there are 4 sig figs

In 101 there are 3 sig figs

When you multiply, the product should be written with the lowest amount of significant figures of the factors. Eg, 1.0 * 2.00 = 2.0

When you add, use the lowest number of deicmal places eg. 1011.000 + 2.0= 1013.0