@Igarizza, the material point basically defines “where the mesh is”.
The snappyHexMesh operation starts with a cartesian grid that fills the complete bounding box (the blue box you need to define). So let’s assume you uploaded a CAD model of the fluid volume of a pipe and you chose the bounding box to be aligned with the pipe model on all sides. Then the mesh initially will be both inside as well as outside the pipe. In one step of this mesh operation. Now in one step of the snappyHexMesh operation, the material point is used to decide which part of the mesh to keep and which not. So if you chose the material point to be inside the pipe, the cells outside the CAD model will be deleted (which is probably what you want in such a setup). However in case you would rather be looking into an external flow analysis of the pipe (just for the sake of explaining this ;)) the material point would need to be outside.
This also makes clear why it’s important to have a closed CAD model (“watertight”) when using the snappyHexMesh operation because otherwise, it might simply mesh both “sides” of the model - inside and outside. Hope I could help!