I'll try to think of it using an example. Say that we conducted a survey one time

It will have a standard deviation and its own mean. The standard deviation will measure how far apart these are.

Now, say that we conducted the survey five times.

We will now have five different means and five different standard deviations. If we only consider the five different means and take the deviation of those 5 different means, then that is our standard deviation of the mean or the standard error.