Standard deviation (SD) tells you about the spread around an average.
If I tell you the averige length of a human male is 1.70 meters - you still have not a clue whether a 1.90 meter male is normal or not.
If you have a SD of 90 % with a size of 20 cm - you know 90 % of all males fit within 1.50 to 1.90 meters
were the SD 10 cm - the same male would no longer be 'normal'
@ how to calculate it - I'm happy to say I forgot