What does a large standard deviation signify?

Standard deviation is basically how much your scores vary from the mean or average score. So if you have a mean of 5 and a standard deviation of 2 it indicates that most of your values are around 5, and if they are not they will usually be +/- 2 units different (between 3 and 7). If you have a large standard deviation it simply means that your data includes a wide range of values. In some cases it may mean that you have an outlier, or an error in your data, in other cases it is normal depending on what you are measuring.

For example if you are taking a sample of peoples ages and you get a mean of 50 and a standard deviation of 20 that would be normal because you can expect ages to range from 0-100. But if you are measuring shoe size and you get a mean of 8 and a standard deviation of 6 you can expect that something is wrong with your data because not many people have size 2 or size 14 shoes.