A question about a derivation in Gali's Monetary Policy book
As mentioned in the comments your question is not possible to answer as it is, but you should know that in macroeconomic models of growth exogenous shocks are measured by log-linearising equations around their steady state and then calculating impulse responses (that is, the magnitude and persistence of a shock to the variables of interest). This must be precisely what your book shows you, and unless you become familiar with macroeconomic modeling this question will hardly be answered any better than it is on your book.