What does this equation mean?

A basic model of directional selection acting on a single locus, of the kind both the field and laboratory results above are ultimately interpreted against, describes how an allele’s frequency changes by one generation under a constant selection coefficient s favoring the allele at frequency p : $p_{t+1} = \frac{p_t (1 + s)}{1 + s\, p_t}$. The equation exposes the model’s real content: selection is strongest while the favored allele is at intermediate frequency, where $p_t(1-p_t)$ is largest, and weakens automatically as the allele nears fixation or loss, purely from the algebra of the denominator — a pattern of decelerating change that shows up empirically in both the Grants’ beak-size tracking and the LTEE’s fitness trajectories, even though the two systems differ in essentially everything else about how the selection coefficient s itself is generated.