“Standard Deviation” is a measure of how spread apart data is: all other things being equal, a data set with a smaller standard deviation is closer to each other; while a data set with larger standard deviation is more spread out.
What this is good for is measuring how far off the “norm” something is – which allows you to compare different things. For example, if I want to look at who is the best athlete right now, I need to compare players in a lot of different games; so my measure is going to be how many standard deviations above average each athlete is. This allows me to compare soccer players to baseball players to American football players to swimmers, and so on.
“Two standard deviations” means two of these standard units of variation.
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