We introduced the notion of weakly unobstructed Lagrangian submanifolds and constructed their potential function ($\mathfrak{PO}$) purely in terms of $A$-model data in [FOOO3]. In this article, we carry out explicit calculations involving $\mathfrak{PO}$ on toric manifolds and study the relationship between this class of Lagrangian submanifolds with the earlier work of Givental [G1], which advocates that the quantum cohomology ring is isomorphic to the Jacobian ring of a certain function, called the Landau-Ginzburg superpotential. Combining this study with the results from [FOOO3], we also apply the study to various examples to illustrate its implications to symplectic topology of Lagrangian fibers of toric manifolds. In particular, we relate it to the Hamiltonian displacement property of Lagrangian fibers and to Entov-Polterovich's symplectic quasi-states