We  introduce Huang-Klemm-Quackenbush's physics approach to the higher genus Gromov-Witten invariants, and  the motivation to consider conifold theory for the quintic threefold. In physics, the conifold theory was originally introduced as the theory for a singular Landau-Ginzburg potential by Ghoshal and Vafa. We will give  known mathematical example which has the gap phenomenon. In the end, we will explain that in the quintic case, the [0,1]-theory introduced in Lecture 1 contains the information which Huang-Klemm-Quackenbush have used and clarify its relation to the conifold gap conjecture.