Following quote is from his second book: "Would it not be wonderful if we could create a single continuum of achievement effects, and locate all possible influences of achievement on this continuum? Adopt an appropriate scale - this was accomplished using effect sizes.
An effect size provides a common expression of the magnitude of study outcomes for many types of outcome variables, such as school achievement." (Hattie, 2015)
Effect size of d=1.0 indicates an increase of one standard deviation on the outcome. A one standard deviation increase is typically associated with advancing children's achievement by two to three years. "Effect Size represents the magnitude of the impact that a given approach has." (Hattie, 2009)
Please also note the 'hinge point' to get the overall context.