Page 41 " the standard deviation of the expected target return can be approximated by the product of three terms: 1) the immunization risk measure, 2) the standard deviation of the variance of the one-period change in the slope of the yield curve,and 3) an expression that is a function of the horizon length only." Can anyone explain to me what are they talking about here specially 1)?

[P40]: the immunization risk measure M2 isâ€¦maturity variance.