What does this equation mean?

$M_t$ is the net machine-mediated contribution: positive where model-assisted re-emission genuinely helps a human originator reach or refine an audience they could not otherwise reach, negative where it substitutes synthetic recombination for content that would otherwise have required, and thereby preserved, a human transmission chain. The mechanism Shumailov and colleagues describe operates by pulling $\rho_t$ itself down as the share of machine-mediated content in what gets re-ingested rises: $\rho_t = \rho_h + (\rho_m - \rho_h)\, s_t, \qquad \rho_m < \rho_h$. where $\rho_h$ is ordinary human-to-human transmission fidelity, $\rho_m$ is the fidelity of a machine-mediated re-emission — empirically lower per the tail-collapse result above — and $s_t$ is the share of newly circulating content, at time t , that is machine-generated rather than human-originated. This decomposition does one useful piece of work: it turns “will culture’s transmission fidelity fall this century” into the answerable, if still hard, empirical question of how $s_t$ moves, and it is that question the scenarios below are actually scenarios about.