What does this equation mean?

A useful way to hold fidelity, bandwidth, and the machine-mediated layer in one place is a simple stock-and-flow account of a corpus’s cultural complexity — the accumulated stock of techniques, texts, and transmissible content a population (human, or now human-plus-model) actually carries forward. In its classical form, without any machine term, this recurrence says that next period’s complexity is this period’s complexity that survived transmission, plus whatever genuinely new content was added, minus whatever was lost to forgetting or to a shrinking population of transmitters: $C_{t+1} = \rho_t C_t + I_t - L_t$. Here $C_t$ is the stock of cultural complexity in circulation at time t ; $\rho_t$ $\in$ [0,1] is the transmission-fidelity coefficient — the share of $C_t$ that survives one round of copying, the same quantity Henrich’s Tasmania case shows collapsing when the effective transmitting population shrinks; $I_t$ is genuinely new human innovation entering the pool; and $L_t$ is loss from forgetting, obsolescence, or population decline. This is the formal skeleton underneath everything in the two sections above: print and the internet acted mainly by raising $\rho_t$ and the volume $I_t$ can reach an audience with; the telegraph and broadcast acted mainly by collapsing the delay between an addition to $I_t$ and its arrival everywhere else in the network.