Operating system: MAC

Slicer version: 4.11

I am using slicer to measure volume of the brain. I typically trace the area I want with draw effect and fill it. I do same thing slide by slide until the whole volume I am interested in is filled. I then use label statistic to get the volume. The program gives me volume and standard deviation. Can anyone please explain what the standard deviation means? I noticed with some measurements I got larger standard deviations than others. What can I do to improve my method and get a more accurate/precise measurements? Thanks