@Thea1,
The standard error is a measure of how confident you are in your statistical parameter. The standard deviation is a property of your sample data set. If you take 20 samples from a distribution and take the mean and standard deviation, you can calculate the standard error in the mean value you calculated. That number is based on the standard deviation. If you take 100 samples, you should get a very similar standard deviation, but now you are much more confident in the mean because you have more data. Your standard error will be lower.