The sample size is the total number of observations in the sample.
Use the sample size to estimate how many observations you need to estimate a parameter within a certain margin of error of the population value with a specified confidence level. When the sample sizes are larger, usually the margins of error are smaller. You want to enough observations in your sample to achieve a small margin of error, but not so many that you waste time and money on unnecessary sampling.
In these results, an analyst wants to estimate the mean voltage of resistors. To achieve a margin of error of 5 with a 95% confidence level, the analyst needs to collect a sample size of 81.