What does this equation mean?

None of this says technological progress stops. It says progress has been purchased, for decades, by feeding an ever-larger population of researchers into a search problem with steadily worsening returns — which is precisely the arrangement that a population no longer growing, and in some read of the evidence above shrinking after mid-century, cannot keep doing indefinitely. If n falls and $\lambda$ and $\phi$ stay where the historical data put them, the semi-endogenous growth relation says the rate of technological advance falls with it. That is not a prediction this article is making outright — Sections 4 and 5 below identify two separate mechanisms that could push back against exactly that conclusion — but it is the default, unmitigated arithmetic, and any claim that a shrinking population is technologically neutral has to explain why one of those mitigating mechanisms will be strong enough to matter.