We're doing stats GCSE at my school, and we learnt Standard Deviation too.
Standard deviation is a way of showing how much data varies from other pieces of data, so how much it deviates.
The formula for standard deviation is slightly complicated, so if you have trouble remembering it, if you have a CASIO calculator- the formula is in the cover. Just follow the instructions on there.
I think it does get you more marks if you can demonstrate that you know how to use it and what it shows, as my sister who didn't do stats- it wasn't introduced when she was in year eleven, hasn't even learned SD in AS level maths yet.