In my course textbook it is written that "approximately 68% of the measurements from a normally distributed set lie within +-1 standard deviation of the mean value". What do they mean by standard deviation of the mean value? They give a definition for "the mean"(of a set of measurements(data)) right before talking about gaussian distribution. Also when they say "68% of the measurements" do they use the word "measurement" as in the meaning of data(e.g. lenght of an object) or set of data(e.g. lenghts of objects)?