What does this equation mean?

with a pairing term commonly written $\delta(A) = \begin{cases} +a_P A^{-1/2} & \text{even-even } N,Z \\ 0 & \text{odd } A \\ -a_P A^{-1/2} & \text{odd-odd } N,Z \end{cases}$. Read as a fitness landscape, each term is a distinct cost or bonus of viability rather than an abstract fitting parameter. The volume term, $a_V$ A , is the baseline: every nucleon in the interior feels attraction from its immediate neighbors, so binding energy grows roughly in proportion to the number of nucleons, the way a population’s raw fitness might scale with sheer numbers before any structural penalty is applied. The surface term, -$a_S$ $A^{2/3}$ , subtracts a cost proportional to surface area, because nucleons at the boundary have fewer neighbors to bind to — a tax on being exposed at the edge of the configuration, heaviest for small A where the surface-to-volume ratio is largest. The Coulomb term, -$a_C$ Z(Z-1)/$A^{1/3}$ , is a pure liability: mutually repelling protons cost binding energy in proportion to roughly $Z^2$ , which is why the landscape eventually tilts against very proton-heavy states no matter how favorable the other terms are. The asymmetry term, -$a_A$ (A-2Z)^2/A , penalizes any departure from N=Z , reflecting the quantum-mechanical cost of filling one nucleon species’ energy levels higher than the other’s while leaving lower levels of the other species empty; it is the term that pulls the landscape’s ridge line away from N=Z as A grows, because the Coulomb term is simultaneously pushing the same ridge toward neutron excess. The pairing term, finally, is a small but structurally important bonus for even-even nuclei and penalty for odd-odd ones, tracing directly to the tendency of like nucleons to couple into paired configurations of lower energy — a bonus with no analogue among the smoothly varying terms, and the term responsible for one of the sharpest features of the landscape’s fine structure, discussed below.