What does this equation mean?

The rapid neutron-capture process runs the opposite calculation. Where the s-process waits for each nucleus to settle before moving on, the r-process floods a small volume of matter with so many free neutrons that a nucleus can capture several before it has any real chance to beta-decay: $\tau_{\beta^-} \gg \tau_n \qquad \text{(r-process)}$. Under these conditions the reaction path runs far from the valley of stability, out toward the neutron drip line, where nuclei are so neutron-rich that adding one more neutron barely binds at all [ 8 ] . Capture continues, isotope after isotope, until it reaches a nucleus at one of the neutron shell closures — the magic neutron numbers 50, 82 and 126 — where the next capture is disfavored and the nucleus instead waits for beta-decay to raise its atomic number before capture resumes. These are the r-process’s “waiting points,” and because so many reaction paths pass through them, the isotopes near them accumulate disproportionate abundance. Once the neutron flux shuts off, every neutron-rich nucleus made this way beta-decays back toward stability, carrying the waiting-point abundance bump with it — which is why the r-process abundance peaks near atomic weight 130 and 195 sit at slightly lower mass than the corresponding s-process peaks: both trace the same magic numbers, approached from opposite sides of the valley of stability [ 7 ] .