What does this equation mean?

Hawkes, O’Connell, Blurton Jones, Alvarez and Charnov turned this ethnographic observation into a formal life-history argument the following year, using Eric Charnov’s dimensionless framework for comparing life-history variables across mammal species. Their comparison of fertility-rate parameters is precise: the annual birth-rate parameter they call b is 0.142 for humans against a range of 0.063 to 0.126 across the great apes they compare against, and the product of age at maturity and that fertility rate, $\alpha$ b , comes out to 2.05 for humans against 0.46 to 0.79 for apes [ 1 ] . Humans combine a maturation schedule that, on Charnov’s dimensionless terms, resembles other apes’ with a fertility rate distinctly higher than theirs — and the paper’s proposed resolution is that this combination becomes selectable only once older, non-reproducing women are provisioning weaned juveniles well enough to let mothers shorten their own interbirth intervals. Longer postmenopausal survival, on this account, is favored because it extends the years over which a woman can keep doing exactly that for her daughters’ children.