What does this equation mean?

Not every element past iron comes from an explosion. Most of the nuclei between strontium and lead that are made by neutron capture rather than fusion are built one careful step at a time, inside ordinary, long-lived stars, by the slow neutron-capture process — the s-process. The name describes the pace: neutron captures happen far apart in time compared to the beta-decay half-lives of the nuclei absorbing them, so each newly captured neutron has time to beta-decay, if it is going to, before the next one arrives. That condition can be written as a simple inequality between two competing timescales: $\tau_{\beta^-} \ll \tau_n \qquad \text{(s-process)}$. where $\tau_n$ is the mean waiting time between successive neutron captures on a given nucleus and $\tau_{\beta^-}$ is that nucleus’s beta-decay half-life. Because the s-process obeys this inequality, its reaction path never strays far from the valley of nuclear stability: a nucleus captures a neutron, and if the product is unstable it decays back toward stability long before it can capture another neutron and be pushed further away [ 7 ] .