What does this equation mean?

Russell Lande’s 1981 paper gave this verbal logic its standard quantitative-genetic form, modelling the joint evolution of a female preference trait and a male display trait under stabilising natural selection on the display and showed that, “despite stabilizing natural selection on males, various types of mating preferences may create a runaway process in which the outcome of phenotypic evolution depends critically on the genetic variation parameters and initial conditions of a population” [ 4 ] . One compact way to see why sits in the coupling itself. Let $\bar t$ be the population mean of a male display trait and $\bar p$ the population mean of the corresponding female preference; write $\beta_t$ and $\beta_p$ for the direct selection gradients acting on each (natural selection pulling $\beta_t$ downward, since the ornament is costly), $h_t^2$ and $h_p^2$ for their heritabilities, and B for the genetic covariance that non-random mating builds between the two traits. The standard Lande-Fisher formalisation of one generation’s change is then a coupled pair: