1 SD from the mean is ADDING and Subtrating the amount if standard deviation from the mean one time. 2 SD from the mean is ADDING and Subtracting TWICE.
for example in this question..
1SD from the mean ranges from 75 to 45, where 75 is within SD above the mean and 45 within 1SD below the mean
2SD=15 TWICE=30 from the the mean, which is 90 to 30, where 90 is within 2 SD above the mean AND 30 is within 2 SD below the mean.
hope this is clear enough, feel free to ask further questions
Sky is the limit