"Sig fig" is an informal abbreviation of the math, science term "significant figures," which is a way of expressing the accuracy of a number or measurement. The rules for significant figures in addition are simple. Because one cannot claim more accuracy than the least accurate number being added, the significant figures in the sum equals that of the number with the smallest amount of significant figures.
For example, the number 1.59 has three significant figures because there are three digits in it defining the number's precision. A number such as 1.1, however, has only two significant digits.
If one adds them together to get 2.69, it is correct arithmetic, but the figure unfairly represents how accurate the number is. The 1.1 shows a level of uncertainty; it could be slightly higher or lower if it represents a measurement with a less-precise device.
In this case, the solution is to round to two significant figures in the sum because the least accurate number added had two significant digits. This would result in an answer of 2.7.
It's easy to confuse the number of significant digits. Just remember that it's a way to express precision. So to the right of the decimal, zeros at the end express additional precision and should count as significant figures. For example, 1.23500 has six significant figures because the two zeros at the end show greater precision.