Dislocations move most readily in slip planes. The reason dislocations move in slip planes is because their high packing density makes dislocation motion easy. The plane chosen here does not have good packing, and is therefore not a good plane for dislocations to move on. Furthermore, high packing planes have lower energy which dislocation motion favors. Dislocation movement is key to the ductility of metals.
Ductility is the ability of a metal to plastically deform.
The smaller the stress that is needed to plastically deform the higher the ductility. If dislocations are allowed to move through slip planes easily then the material will deform plastically. In planes where dislocation can not move dislocations become tangled and the material will resist plastic deformation. The plot shown is important because it shows the areas where dislocations could move through. The higher the plot the higher the energy.