The standard deviation is the approximate variation between the absolute middle of the data set (the median) and the highest and lowest numbers. This often sets a bell curve, which is what a lot of classes use now to grade on. The 100% (or whatever happens to be the highest) is the highest grade, so it’s set at the furthest right of the graph. The 0% (or lowest grade) sits at the furthest left. Everything else falls between those grades, plus or minus some from the median 50% (varies based on other grades).
The standard deviation has a bit of a complicated formula. It’s too complicated to go into here, but basically, it’s a square root of all the data put together in a convoluted way. A quick search online will yield the equation and what goes in each spot.