What does this equation mean?

The first half of the argument is purely elastic and predates Griffith. An elliptical hole in a stressed plate does not merely remove load-bearing material; it redistributes the stress that would have passed through that material into the region around the hole’s sharpest curvature. For an elliptical flaw of half-length a and tip radius of curvature $\rho$ in a plate under remote tension $\sigma_\infty$ , the peak stress at the tip is approximately $\sigma_{\max} = \sigma_\infty\left(1 + 2\sqrt{\frac{a}{\rho}}\right)$. Three things follow. First, the concentration depends on the flaw’s aspect ratio, not on its absolute size — a long shallow scratch and a short sharp one can concentrate stress equally. Second, as the tip sharpens toward an atomically sharp crack, $\rho$ $\to$ 0 and the predicted peak stress diverges. Third, and most usefully, a designer cannot avoid stress concentration by making the part thicker; the concentration factor is geometric.