An incoming photon produces a virtual electron, which absorbs the second photon and produces the outgoing electron-positron pair. Related to pair annihilation by time reversal and to Compton scattering by crossing.
The computational chain
From diagram to number—every step of the amplitude calculation.
The diagram
An incoming photon produces a virtual electron, which absorbs the second photon and produces the outgoing $e^+e^-$ pair. Related to pair annihilation by time reversal and to Compton scattering by crossing: $s_{\text{Compton}} \to t_{\text{production}}$.
Crossing relation
$$|\mathcal{M}(\gamma\gamma \to e^+e^-)|^2 = |\mathcal{M}(e^+e^- \to \gamma\gamma)|^2\bigg|_{s\leftrightarrow t}$$
The squared amplitude is identical to pair annihilation with Mandelstam variables relabeled. The cross section differs because the flux factor and phase space depend on the initial state.